Display, instrument panel, optical system and optical instrument

ABSTRACT

A multiple depth display provided for displaying images at different depths comprises a single display device ( 61 ), for displaying all of the images. An optical system ( 62, 63, 64 ) is disposed in front of the display device ( 61 ). The optical system comprises first and second spaced-apart partial reflectors ( 62, 63 ) and polarization optics ( 64 ) for providing first and second light paths for first and second images or sequences of images displayed by the device ( 61 ). The first light path ( 65 ) comprises partial transmission through the first reflector ( 62 ), partial reflection from the second reflector ( 63 ), partial reflection from the first reflector ( 62 ), and partial transmission through the second reflector ( 65 ) towards a viewing region.

TECHNICAL FIELD

The present invention relates to a display. Such a display may be used,for example, to provide an impression of depth or changed depth. Such adisplay may, for example, be used in information display applicationsincluding computer-aided design, games and television and inapplications where warnings or other messages are required to stand outfrom a background. The present invention also relates to an instrumentpanel including such a display. The present invention further relates toan optical system and to an optical instrument including such a system.

BACKGROUND ART

It is known for vehicles, such as automobiles and aircraft, to includean electronic display providing an image of, for example, an instrumentcluster for replacing discrete mechanical or electric dials. However,such displays generally provide limited realism because of theirinability to produce images at different depths with respect to thedisplay apparatus. In addition to limiting the realism of such displays,the inability to produce images at different depths reduces thevisibility or intelligibility of the images. Although stereoscopic andautostereoscopic displays are known and can produce an impression of athree-dimensional image, such displays do not produce an impression oftrue depth, being unable to reproduce focusing information correctlyFurther, such displays may have limited freedom of viewing position andmay result in user confusion and even eye strain and headaches.

FIG. 1 of the accompanying drawings illustrates a display of the typedisclosed in U.S. Pat. No. 4,736,214 for displaying background andforeground images with different image depths. The display comprises aprojector 1 for projecting images carried by a projection film 2 witheach frame being divided to provide a background image 3 and aforeground image 4. The projector 1 projects these images simultaneouslyfor each frame onto an optical system comprising a rear projectionscreen 5, mirrors 6 and 7 and a partially transmitting mirror 8. Thebackground and foreground images are projected via different lengthoptical paths in order to produce a motion picture with two depthplanes. Although such an apparatus is capable of showing images withdifferent depths to an audience 9, it is of limited application becauseof its relatively large size and its use of relatively expensiveequipment.

FIG. 2 of the accompanying drawings illustrates a display of the typedisclosed in WO 9942889, WO 03040820, WO 04001488, WO 04002143 and WO04008226. This type of display is of dual-panel construction andcomprises a backlight 10 overlaid by spatial light modulators 11 and 12.The spatial light modulators 11 and 12 modulate light from the backlight10 with a pair of images or sequences of images so as to display theimages or sequences at different depths. However, such an arrangementhas several disadvantages. For example, the spatial light modulators 11and 12 may have the same regular patterns of black masks which result inthe appearance of Moiré fringes, requiring further optical elements 13in order to reduce the appearance of such fringes. Also, the use of two(or more) spatial light modulators results in very low lighttransmission so that a very bright backlight 10 is required in order toachieve the necessary or desired image brightness for viewing.

Such an arrangement is “light-subtractive” so that, for example, pixelsin the first modulator 11 must be “on” or transmissive in order forpixels in the line of sight in the second modulator 12 to be visible toa viewer. Thus, a light object cannot be shown on a dark background.Also, as light has to pass through the two spaced modulators 11 and 12,parallax effects can occur at image boundaries.

The use of multiple spatial light modulators substantially increases thecost of such a display as compared with conventional displays usingsingle spatial light modulators. In order to increase the number ofdepth planes, the number of spatial light modulators must be increasedand this results in a linear increase in cost, and an experientialdecrease in brightness, with the number of depth planes. Further, suchan arrangement requires synchronised control of multiple spatial lightmodulators.

U.S. Pat. No. 2,402,9626 also discloses a multiple panel display of asimilar type intended for use in a wagering gaming apparatus.

EP 01059626 and EP 0454423 disclose multiple layer displays having fixedelectrode patterns for use in specific applications, such as in watchesor in hand-held games. EP 1265097 discloses a display for an automotiveinstrument cluster comprising a matrix-addressable display overlaid witha patterned display for showing specific vehicle functions. Suchdisplays have the same disadvantages as the multiple panel displaysdescribed above and, in addition, are capable of showing only limitedimages as determined by the electrode patterns.

EP 1093008, JP 0226211, WO 0911255, JP 62235929 and US 22105516 disclosevolumetric displays based on multiple layer scattering andpolariser-free display panels. Such displays are intended to improve thebrightness of the displayed images compared with light-absorbing displaypanels. However, displays of this type have various disadvantages. Forexample, a dark state is produced by a non-scattering state so thatlight is transmitted to the environment. This is undesirable in manyapplications, such as in automotive displays particularly duringnight-time driving. Also, such multiple displays are relativelyexpensive. Further, displays of this type generally have relatively slowswitching times and are unsuitable for use throughout wide temperatureranges, for example as may be found in an automotive environment.

FIG. 3 of the accompanying drawings illustrates a known type oftime-sequential projection volume display, for example as disclosed inU.S. Pat. No. 4,333,715, US 22163482 and U.S. Pat. No. 4,670,744. Theimages for the different planes are displayed sequentially by aprojector 15 and projected towards a plurality of projection screens 16.The projection screens 16 are of an active type and are enabled one at atime in synchronism with projection by the projector 15 of the imageintended to be viewed at the location of the screen. The projectionscreens 16 are of reflective or scattering type when switched on and aresubstantially transparent when switched off. However, such a display isof limited use because of the large volume which it requires. Also, sucha display is inconvenient because of the need to synchronize activationof the projection screens with the images projected by the projector.

The DaimlerChrysler F500 Mind Car research vehicle shown at the 2003Tokyo motor show disclosed an instrument cluster which overlaid, bymeans of a half-silvered mirror, a standard instrument cluster and aliquid crystal display (LCD) panel. However, such an arrangementrequires substantial volume in order to accommodate two displays whichmust be disposed at an angle with respect to each other. Also, asdescribed hereinbefore, the use of multiple displays makes such a systemrelatively expensive.

FIGS. 4 to 6 of the accompanying drawings illustrate displays of thetype disclosed in WO 09810584. The display shown in FIG. 4 comprises ahousing 20 containing a beam-combining element in the form of apartially reflective optical element 21. The housing 20 has a viewingaperture 22 and apertures in which are located display devices 23 and 24for displaying foreground and background images, respectively. Theoptical paths from a viewer to the display devices 23 and 24 aredifferent so that the device 23 appears at its actual location whereasthe device 24 appears behind the device 23 to provide a virtualbackground image 25.

The display shown in FIG. 5 of the accompanying drawings comprises asingle display device 30 divided into relatively large regions fordisplaying interlaced foreground and background images as illustrated at31 and 32. Light from each of the foreground elements 31 passes througha partially reflecting mirror 33 and an array of optical expansionelements 34 directly to a viewing region whereas light from each of thebackground elements 32 is reflected by a mirror 35 and the partiallyreflecting mirror 33 so as to have a longer light path to the viewingregion.

FIG. 6 of the accompanying drawings illustrates another displaycomprising a projector 40 and a rotating rod 41 on which are mountedfirst and second projection screens 42 and 43. The projector projectsfirst and second images or sequences of images in synchronism with thescreens 42 and 43, respectively, being in front of the projector.

The display illustrated in FIG. 4 has the disadvantages of occupying arelatively large volume and being relatively expensive because of theuse of multiple display devices. The display shown in FIG. 5 has thedisadvantage that the interlaced sections of the display device 30 arerelatively large so that the additional expansion elements 34 arerequired in order for the image to fill an entire display region. Thepresence of such elements results in reduced freedom of movement of aviewer because of the f-number of the elements 34. The elements 34require exact alignment with the regions of the display device 30 andthis is inconvenient during manufacture and increases the cost. Twomirror elements are required for each background section of the displaydevice and this increases the size and cost of manufacture. Anyaberration in the elements 34 results in image distortion as a viewermoves relative to the display, even if perfect compensation is providedfor on-axis viewing.

The display shown in FIG. 6 of the accompany drawings occupies arelatively large volume and requires a mechanical system to providedifferent image depths. Also, this display has the inconvenience ofrequiring means for providing synchronization between the rotaryposition of the mechanical assembly and the projected images.

US 2005/0156813 discloses a display of the type illustrated in FIG. 7 ofthe accompanying drawings. The display comprises an LCD panel 45 havinga portion 46 for displaying a foreground image and a portion 47 fordisplaying a background image. The panel 45 supplies light modulatedwith both images having a polarization direction in the plane of FIG. 7of the accompanying drawings. Light from the portion 46 passes through areflective polariser 48 to a viewing region of the display.

Light from the portion 47 passes through a retarder 49, which changesthe polarization direction of light by 90° so that it is perpendicularto the plane of FIG. 7 of the accompanying drawings. The resulting lightis reflected by a mirror 50 towards the reflective polariser 48 suchthat the reflected light is again reflected by the polariser 48 towardsa viewing region.

Although such a display can provide different image depths from a singleLCD panel, separate portions of the panel are required to form the twoimages so that, in order to create a multiple depth image of a givensize, a much larger display panel is required. Also, the presence of themirror 50 greatly restricts the viewing angle of the display. This alsolimits the orientations of the image planes, which cannot beperpendicular to the viewing direction.

DISCLOSURE OF INVENTION

According to first aspect of the invention, there is provided an opticalsystem for providing a first light path which is longer than a physicallength of the system, the optical system comprising first and secondspaced-apart partial reflectors and providing the first light path for afirst light incident on the first reflector, the first light pathcomprising at least partial transmission through the first reflectortowards the second reflector, at least partial reflection from thesecond reflector towards the first reflector, at least partialreflection from the first reflector towards the second reflector, and atleast partial transmission through the second reflector, the opticalsystem being arranged substantially to prevent emission from the secondreflector of the first light not reflected during reflection by thefirst and second reflectors, wherein light incident on the secondpartial reflector for the first time does not leave the optical system.

It is thus possible to provide a relatively simple optical system whichis relatively inexpensive to manufacture. The optical system is capableof providing a light path which is longer than the physical length ofthe optical system. Such an arrangement has many applications, includingproviding an image in a display which is more remote than an imagedisplaying device and shortening the length of optical instruments.

The optical system may be arranged to change the polarization of thefirst light during passage along the first path. The optical system maybe arranged to change the polarization of the first light during passagealong the first path between incidence on the second reflector andreflection from the first reflector.

The optical system may be arranged to provide a second light path oflength different from that of the first path. The second light path maycomprise at least partial transmission through the first reflectortowards the second reflector and at least partial transmission throughthe second reflector. The optical system may be arranged substantiallyto prevent emission from the second reflector of the second light nottransmitted by the second reflector. The optical system may beswitchable between a first mode, in which the first light propagatesalong the first light path, and a second mode in which light propagatesalong the second light path.

It is further possible to provide an optical system having two lightpaths of different lengths with at least one being different from thephysical length of the system. Again, the system is relatively compactand inexpensive to manufacture. When used with a direct view displaydevice, it is possible to provide a direct view display having imageslocated at different distance from a viewer with at least one of theimages being displaced in the depth direction from the physical positionof the display device.

The first and second reflectors may substantially plane.

The first and second reflectors may be substantially parallel.

The first and second reflectors may comprise a reflective linearpolariser and a partially transmissive mirror, respectively, and theoptical system may comprise: a circular polariser with the secondreflector disposed between the first reflector and the circularpolariser; a quarter wave plate disposed between the first and secondreflectors; and a switchable half wave plate disposed between the firstreflector and the circular polariser.

The first and second reflectors may comprise a partially transmissivemirror and at least one reflective circular polariser, respectively. Theoptical system may comprise a quarter wave plate. The optical system maycomprise a switchable half wave plate. The first and second reflectorsmay comprise reflective polarisers and the optical system may comprise aswitchable directional half wave plate.

The first and second reflectors may comprise reflective linearpolarisers and the optical system may comprise a Faraday rotator and aswitchable half wave plate.

According to a second aspect of the invention, there is provided anoptical instrument comprising a system according to the first aspect ofthe invention.

The instrument may comprise at least one refractive, reflective ordiffractive element having optical power. For example, the instrumentmay comprise a telescope, a monocular, a pair of binoculars or a camera.

According to a third aspect of the invention, there is provided adisplay comprising a display device for modulating a first light with afirst image or sequence of images and an optical system arranged toincrease the perceived depth of location of the first image or sequence,the optical system comprising first and second spaced-apart partialreflectors and providing a first light path for the first light from thedevice to a viewing region, the first light path comprising at leastpartial transmission through the first reflector towards the secondreflector, at least partial reflection from the second reflector towardsthe first reflector, at least partial reflection from the firstreflector towards the second reflector, and at least partialtransmission through the second reflector towards the viewing region.

The optical system may be arranged substantially to prevent transmissionto the viewing region of the first light not reflected during reflectionby the first and second reflectors.

The optical system may be arranged to change the polarization of thefirst light during passage along the first path. The optical system maybe arranged to change the polarization of the first light during passagealong the first path between incidence on the second reflector andreflection from the first reflector.

The device may be arranged to modulate a second light with a secondimage or sequence of images and the optical system may be arranged toprovide a second light path from the device to the viewing region oflength different from that of the first path to provide a perceiveddepth of location of the second image or sequence different from that ofthe first image or sequence. The second light path may comprise at leastpartial transmission through the first reflector towards the secondreflector and at least partial transmission through the second reflectortowards the viewing region. The optical system may be arrangedsubstantial to prevent transmission to the viewing region of the secondlight not transmitted by the second reflector.

The display may be switchable between a first mode displaying the firstimage or sequence and a second mode displaying the second image orsequence to change the perceived image location depth.

The display may be arranged to display the first and second images orsequences simultaneously or time-sequentially to give the appearance ofone of the first and second images or sequences overlaid above the otherof the first and second images or sequences. The display may comprise animage generator for generating image data for the other image orsequence representing a scale and for the one image or sequencerepresenting a pointer. The image data for the one image or sequence mayrepresent a visible warning.

The first and second reflectors may be substantially plane.

The first and second reflectors may be substantially parallel. The firstand second reflectors may be substantially parallel to a display surfaceof the device.

The device may be a light-emissive device.

The device may comprise a transmissive spatial light modulator. Themodulator may comprise a liquid crystal device.

The first and second reflectors may overlie substantially the whole ofan image displaying region of the device.

The first and second reflectors may comprise a reflective linearpolariser and a partially transmissive mirror, respectively, disposedbetween the device and a circular polariser and the optical system maycomprise a quarter wave plate disposed between the first and secondreflectors and a switchable half wave plate disposed between the firstreflector and the circular polariser.

The first and second reflectors may comprise a partially transmissivemirror and at least one reflective circular polariser, respectively. Theoptical system may comprise a quarter wave plate. The optical system maycomprise a switchable half wave plate.

The first and second reflectors may comprise reflective polarisers andthe optical system may comprise a switchable directional half waveplate.

The first and second reflectors may comprise reflective linearpolarisers and the optical system may comprise a Faraday rotator and aswitchable half wave plate.

The first and second reflectors may comprise a partially transmissivemirror and a reflective linear polariser, respectively, and the opticalsystem may comprise a quarter wave plate disposed between the first andsecond reflectors and a patterned retarder or polarization rotatordisposed between the first reflector and the device.

The display may comprise a linear exit polariser for or of the device.

The display may comprise a collimated backlight and the optical systemmay comprise a diffuser. The partially transmission mirror may comprisea mirror having an array of apertures and the diffuser may comprise anarray of lenses aligned with the apertures. The lenses may be converginglenses.

The patterned retarder or rotator may be switchable to a uniformunpatterned state.

The patterned retarder or rotator may comprise a patterned quarter waveplate.

The patterned retarder or rotator may comprise a uniform quarter waveplate and a patterned half wave plate or patterned 90° polarizationrotator.

The patterned retarder or rotator may comprise a liquid crystal cell.The liquid crystal cell may comprise a patterned electrode arrangement.

The device may be scan-refreshed and the optical system may comprise asegmented switchable polarization-affecting element, whose segments arearranged to be switched when associated portions of an image displayedby the device have been refreshed.

The modulator may be scan-refreshed and the device may comprise abacklight arranged to be illuminated between consecutive pairs of framerefreshes.

According to a fourth aspect of the invention, there is provided aninstrument panel for a vehicle including a display according to thethird aspect of the invention.

It is thus possible to provide a display which is relativelyinexpensive. The use of multiple display panels is generally unnecessaryand this contributes to the reduced cost. This also avoids therelatively low light through-put of known multiple display devicearrangements. Eye-strain associated with 3D displays creating astereoscopic illusion of depth can be avoided by producing one or morevirtual images. Also, the display may be visible throughout a relativelywide viewing region, allowing considerable viewing freedom for a viewer.

When used for multiple image or multiple sequence display purposes, suchan arrangement is “additive” so that a light object may appear in frontof a dark background. Further, Moiré effects are generally reduced incomparison with various known multiple display panel arrangements. Lightnot used to display images or image sequences may be absorbed within thedisplay instead of being transmitted into the environment.

Such an arrangement requires no moving parts and, in many embodiments,no alignment between optical elements is required during manufacture.Such an arrangement may be used with emissive or transmissive displaydevices.

It is also possible to provide an optical system which has a light pathwhich is longer than the physical length of the system. Such a systemmay be used, for example, in optical instruments and allows suchinstruments to be more compact. In addition to reducing the lengths ofsuch instruments, the system does not have to be of greater width. Thelight path does not have to be deviated sideways but is effectively“folded on itself” so that the length may be reduced without increasingthe width and without requiring two or more light path portions whichare spaced apart sideways.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 2 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 3 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 4 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 5 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 6 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIG. 7 is a cross-sectional diagrammatic view showing an example ofconventional multiple image depth display;

FIGS. 8( a) and 8(b) are diagrams illustrating a display constituting ageneralized embodiment of the invention and illustrating a depth-addingmode and a “no-depth” mode, respectively;

FIG. 9( a) to FIG. 9( c) are diagrams illustrating a displayconstituting a first embodiment of the invention;

FIGS. 10( a) to 10(d) are diagrams illustrating the operation of thefirst embodiment;

FIG. 11 is a diagram illustrating a display constituting a furthermodified first embodiment of the invention;

FIGS. 12( a) and 12(b) are diagrams illustrating the structure andoperation of a display constituting a second embodiment of theinvention;

FIGS. 13( a) to 13(c) are diagrams illustrating a display (FIGS. 13( a)and 13(b)) and a modified display (FIG. 13( c)) constituting thirdembodiments of the invention;

FIGS. 14( a) and 14(b) are diagrams illustrating the structure andoperation of a display constituting a fourth embodiment of theinvention;

FIGS. 15( a) and 15(b) are diagrams illustrating colour filterperformance and a modified display constituting another example of thefourth embodiment, respectively;

FIG. 16 is a diagram illustrating a display constituting a fifthembodiment of the invention;

FIGS. 17( a) and 17(b) are diagrams illustrating a display constitutinga sixth embodiment of the invention;

FIGS. 18( a) and 18(b) are diagrams illustrating operation of thedisplay of FIGS. 17( a) and 17(b) in a first depth mode;

FIGS. 19( a) and 19(b) are diagrams illustrating operations of thedisplay of FIGS. 17( a) and 17(b) in a second depth mode;

FIGS. 20( a) and 20(b) are diagrams illustrating a display constitutinga seventh embodiment of the invention;

FIGS. 21( a) and 21(b) are diagrams illustrating modifications of thedisplay shown in FIGS. 20( a) and 20(b);

FIG. 22 is a diagram illustrating a further modification of the displayof FIGS. 20( a) and 20(b);

FIGS. 23( a) to 23(e) are diagrams illustrating examples of an opticalelement for use in interlaced image embodiments of the invention;

FIGS. 24( a) and 24(b) are diagrams illustrating a display constitutingan eighth embodiment of the invention;

FIGS. 25( a) and 25(b) are diagrams illustrating a display constitutinga ninth embodiment of the invention;

FIG. 26 is a diagram illustrating a display constituting a ninthembodiment of the invention;

FIGS. 27( a) and 27(b) are diagrams illustrating a modified form of thedisplay shown in FIGS. 25( a), 25(b) and 26.

FIG. 28 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 29 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 30 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 31 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 32 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 33 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 34 is a diagram illustrating further modified forms of the displayshown in FIGS. 25( a), 25(b) and 26;

FIG. 35 is a graph of displayed brightness against greyscale or valueillustrating a modified gamma function;

FIG. 36 is a diagram illustrating LCD pixel arrangements suitable foruse in the displays;

FIG. 37 is a diagram illustrating LCD pixel arrangements suitable foruse in the displays;

FIG. 38 is a diagram illustrating an application of displaysconstituting embodiments of the invention;

FIGS. 39 and 40 are diagrams illustrating operation of displaysconstituting embodiments of the invention;

FIGS. 41( a) and 41(b) are diagrams illustrating a modification whichmay be applied to some embodiments of the invention;

FIGS. 42( a) through 42(d) are diagrams illustrating crosstalkcorrection which may be applied to some embodiments of the invention;

FIGS. 43( a) and 43(b) are diagrams illustrating an application of thedisplays for simulating a control button;

FIG. 44 is a diagram illustrating a known type of astronomicaltelescope;

FIG. 45 is a diagram illustrating an astronomical telescope constitutinga tenth embodiment of the invention;

FIG. 46 is a diagram illustrating a terrestrial telescope constitutingan eleventh embodiment of the invention;

FIG. 47 is a diagram illustrating a known type of digital camera; and

FIG. 48 is a diagram illustrating a digital camera constituting atwelfth embodiment of the invention.

BEST MODE FOR CARRYING OUT INVENTION

FIGS. 8( a) and 8(b) illustrate the construction of a display and twooperational modes of the display. The display comprises a backlight 60disposed behind a spatial light modulator (SLM) in the form of a liquidcrystal device (LCD) 61. First and second partial reflectors 62 and 63are disposed in front of the LCD 61 (on the viewer side thereof) withpolarization-modifying optics 64 disposed between the reflectors 62 and63. The reflectors 62 and 63 are separated from each other by anappropriate spacing for producing a depth-shifted image and are parallelto each other and to an image surface of the LCD 61. For example, thepartial reflectors 62 and 63 may be arranged to reflect one polarizationstate of light and to transmit the orthogonal state or may be partiallyreflecting mirrors (or combinations of reflecting elements) of someother type. The polarization optics 64 are arranged to change at leastone polarization state of light passing in either or both directionsthrough the optics 64.

The elements 61 to 64 are arranged such that light from first and secondimages or sequences of images displayed by the LCD 61 travels alongdifferent light paths towards an extensive viewing region where one ormore viewers may be located. Examples of these light paths for the firstand second images or sequences are illustrated at 65 and 66,respectively. For the first image or sequence, light is at leastpartially transmitted by the first reflector 62 towards the secondreflector 63. The second reflector 63 reflects at least part of thislight towards the first reflector 62, which reflects at least part ofthe incident light back towards the second reflector 63. The secondreflector 63 transmits at least part of the reflected light to theviewing region so that light encoded with the first image or sequencefollows a “zig-zag” path before reaching a viewer. The display isarranged such that light encoding the first image or sequence does notpass directly by transmission through the reflectors 62 and 63 to theviewer.

As illustrated by the light path 66, light encoded with the second imageor sequence is transmitted at least partially by the reflectors 62 and63 so as to follow an essentially direct path to the viewing region. Asa result of the different paths 65 and 66, in particular their differentlengths, the second image or sequence appears substantially at thelocation of the LCD 61 whereas the first image or sequence is shifted indepth so as to appear at the location 67. The display thus acts as adual-depth display to allow a viewer to see images in different depthplanes.

Whether light follows the path 65 or the path 66 may be determined in anumber of different ways. Examples of these include using the paths 65and 66 at different times or by different colours or by light emergingfrom different parts of the LCD 61.

Such a display may be operated in ways other than as a dual-depthdisplay (in which a viewer can see images in the different depth planesat effectively the same time). For example, the display may be operatedsuch that light follows the path 65 and does not follow the path 66. Inthis case, the display acts as a depth-shifting display so that imagesappear to come from a plane further away from the viewer than the LCD61. Such an arrangement would allow the display to show images whichappear further away and at a location where it is not be possible orconvenient to mount the display.

The display may also be operated in such a way as to switch between thedifferent depth image planes. For example, a gaming machine may show animage which appears at the image plane 67 during normal operation.However, when a player wins a prize, the depth plane may be switched sothat the image appears to leap forwards.

Where the display is operated as a dual-depth display, images for thedifferent planes may be displayed time-sequentially. Thus, the LCD 61alternates between displaying the first and second images and, ifnecessary, the polarisation optics 64 are switched in synchronism sothat light from the first images follows the light path 65 whereas lightfrom the second images follows the light path 66. Provided switchingbetween images is performed sufficiently rapidly to avoid the visibilityof flicker, a viewer sees the images at their intended different depthplanes.

This type of operation has the advantage that all images are displayedin full colour and at the full resolution of the LCD 61. However, thistype of operation requires that the LCD 61 be capable of operating at asufficiently high frame rate, for example of the order of 100 Hz ormore, to eliminate the appearance of flicker. LCDs do exist foroperating at such frame rates. However, the display is not limited tothe use of LCD SLMs and other suitable devices may be used includinglight-emissive devices as well as light-attenuating devices withbacklights. For example, other types of display devices which may beused include cathode ray tubes, plasma display devices, projectiondisplay systems and organic light emitting diode (OLED) display devices.

As an alternative, a dual-depth display may be based on polarizationoptics and reflectors which have a different effect on light ofdifferent wavelengths. For example, the reflectors 62 and 63 and theoptics 64 may be arranged such that light of some wavelengths, forexample red light, follows the path 66 whereas light of some otherwavelengths, for example blue and green light, follows the path 65. Aviewer would then see images where different colours appear in differentdepth planes.

This type of operation does not require the high frame rates oftime-sequential displays. Also, images are displayed in the full spatialresolution of the display device, such as the LCD 61. However, fullcolour images cannot be shown in the different depth planes unless thewavelength bands are chosen to be sufficiently narrow for each primarycolour (red, green, blue) to be split into two bands with one followingthe light path 65 and the other following the light path 66.

In another mode of operation suitable for a dual-depth display, thefirst and second images are spatially interlaced on the LCD 61. Thereflectors 62 and 63 and the optics 64 are then arranged so that lightfrom the pixels displaying the first images follows the path 65 whereaslight from the pixels displaying the second images follows the path 66.The images may be interlaced in rows or columns on the LCD 61. Such anarrangement does not require a high frame rate and displays images infull colour. However, the resolution of the images is less than thebasic spatial resolution of the LCD 61; for a dual-depth display, theresolution in which each image is displayed is half the basic resolutionof the LCD 61.

FIGS. 9( a) and 9(b) illustrate a display of the type shown in FIGS. 8(a) and 8(b). In this embodiment, the first partial reflector 62comprises a reflective polariser, which transmits one linearpolarization state of light and reflects the orthogonal state. Such apolariser may, for example, comprise a wire-grid polariser of the typemanufactured by Moxtek Inc. or a DBEF (directional brightness enhancingfilm) manufactured by 3M. Where the LCD 61 requires an exit polariser,the reflective polariser 62 may be used as the exit polariser or may beprovided in addition to the exit polariser.

A fixed quarter wave plate 68 is disposed above the reflective polariser62 and is oriented so as to convert between linearly polarized light andcircularly polarized light. Although the quarter wave plate 68 maysimply comprise a film of birefringement material of the appropriatethickness, such a film performs the quarter wave function accurately foronly a single wavelength. The quarter wave plate 68 may be formed from aplurality of birefringement layers in order to provide an element whichacts as an ideal quarter wave plate for a range of wavelengths acrossthe visible spectrum. Such films are available from Polatechno Limitedof Japan or from Sumitomo Chemical Corporation of Japan.

A switchable half wave plate 69 is disposed above the quarter wave plate68. Such a switchable halfwave plate may comprise a liquid crystal cellwhich is capable of being switched on and off electrically. Examples ofsuch cells which are suitable for this application include thevertically aligned nematic (VAN) cell, the Freedericksz cell, and the picell or optically compensated birefringence (OCB) cell. Such cells arewell known and are disclosed in standard reference publications onliquid crystal displays, such as “Liquid Crystal Displays AddressingSchemes and Electro-Optic Effects”, Ernst Lueder, Wiley-SID Series inDisplay Technology 2001. Pi cells are well-suited to this applicationbecause of their ability to switch quickly between on and off states.

A spacer in the form of a layer of glass 70 or other transparentmaterial having substantially no effect on the polarisation state oflight passing therethrough is disposed above the switchable half waveplate 69. The spacer 70 is optional and may be provided in order toachieve the desired increase in apparent depth.

The second partial reflector 63 comprises a partially reflecting andpartially transmitting mirror. The mirror 63 is illustrated as “50%”mirror which reflects approximately half of the incident light andtransmits approximately half of the incident light. However, thefraction of light transmitted or reflected may be chosen in order toachieve a desired relative brightness of the images displayed at thedifferent depths.

The mirror may be made by coating a thin layer of a metal such asaluminum on a transparent substrate or may comprise a coating oftransparent dielectric layers (a dielectric mirror). Partial reflectionmay be achieved either by making the reflecting layer uniformlypartially transparent, or by using a completely reflecting mirror withtransparent gaps or holes. If these holes or gaps are on a scale smallerthan those visible to the eye, then the hole or gap pattern will not bevisible and the mirror will appear partially reflecting and partiallytransparent.

For mirrors constructed from metal layers, the use of holes or gaps maybe preferable to a uniform partial reflector for two reasons: it may bedifficult to control layer thickness accurately so as to achieve areproducible and uniform reflectivity in a uniform layer; and dependenceof reflectivity on polarisation state may be weaker in a mirror withholes than in a uniformly partially reflecting mirror.

A circular polariser 71 is disposed above the mirror 63. The circularpolariser transmits left-handed circularly polarized light and absorbsright-handed circularly polarized light. The order of the opticalelements may be varied without changing the operation of the display.For example, the glass layer 70 may be disposed anywhere between thereflective polariser 62 and the partially-reflecting mirror 63. Also,the positions of the quarter wave plate 68 and the switchable half waveplate 69 may be exchanged.

Operation of the display in the depth-shifting mode is illustrated inFIGS. 10( a) and 10(b). The polariser 62 has a transmission axis 73oriented in the plane of the drawing and the quarter wave plate 68 has afast axis 74 oriented at 45° to the transmission axis 73. Light from theLCD 61 is polarized with its electric field vector in the plane of thedrawing (<<) and thus passes through the reflective polariser 62. Thequarter wave plate 68 converts the linearly polarized light toright-handed circularly polarized light R, which is partially reflectedand partially transmitted by the mirror 63. In this mode, the half waveplate 69 is switched off and is therefore omitted from (a) in FIG. (10).

The transmitted light is absorbed and therefore blocked by the circularpolariser 71 whereas the reflected light is converted to left-handedcircularly polarized light L. The quarter wave plate 68 converts thelight to a state which is linearly polarized in a directionperpendicular to the plane of the drawing (Ä). This light is reflectedby the reflecting polariser 62 and is converted to left-handedcircularly polarized light by the quarter wave plate 68. The portion ofthis light which is transmitted by the mirror 64 is also transmitted bythe circular polariser 71 and propagates towards the viewing region ofthe display. The portion of light reflected by the mirror 63 isconverted to right-handed circularly polarized light, which is convertedby the quarter wave plate 68 to light which is linearly polarized in adirection parallel to the plane of the drawing. This light istransmitted back into the LCD 61 by the polariser 62.

In this mode of operation, the only light which passes to the viewingregion is that which has been reflected by the partial reflectors 62 and63. Thus, as described hereinbefore, a viewer sees an image of the LCD61 at the position 67 as illustrated in FIG. 9( a). The image iseffectively displaced “downwards” from its actual position byapproximately twice the separation between the partial reflectors 62 and63, the actual displacement being also determined by the refractiveindices of various elements of the display.

FIGS. 10( c) and 10(d) illustrate the operation of the display with thehalf wave plate 69 switched on. As illustrated at in FIG. 10( d), thefast axis of the half wave plate 69 may have any orientation. However,in practice, for example because of imperfections in the opticalelements used in the display or because of dependence of theirproperties on viewing angle, there may be a preferred orientation. Inthis application, the half wave plate coverts between right-handedcircularly polarized light R and left-handed circularly polarized lightL.

As illustrated in FIG. 10( c), light which is linearly polarized withits direction of polarization in the plane of the drawing is transmittedby the reflecting polariser 62 and converted to right-handed circularlypolarized light by the quarter wave plate 68. The half wave plate 69converts this to left-handed circularly polarized light which is partlytransmitted and partly reflected by the mirror 63. The transmitted lightpasses through the circular polariser 71 towards the display viewingregion. The reflected light is converted to right-handed circularlypolarized light, which the half wave plate 69 converts to left-handedcircularly polarized light. The quarter wave plate 68 converts this tolight which is linearly polarized in a direction perpendicular to theplane of the drawing. This light is reflected by the reflectingpolariser 62 and converted to left-handed circularly polarized light bythe quarter wave plate 68.

The half wave plate 69 converts this light to right-handed circularlypolarized light, part of which is transmitted and part of which isreflected by the mirror 63. The transmitted light is absorbed by thecircular polariser 71 and so is blocked and prevented from beingtransmitted to the viewing region. The reflected light is converted toleft-handed circularly polarized light, which is converted toright-handed circularly polarized light by the half wave plate 69. Thequarter wave plate 68 converts this to light which is linearly polarizedin a direction parallel to the plane of the drawing and the reflectingpolariser 62 transmits this light back to the LCD 61.

In order to operate the display illustrated in FIGS. 9 a, 9 b and 10 ato 10 d in the dual-depth mode, the display is operatedtime-sequentially. Thus, images which are to be visible with and withoutdepth-shifting are alternately displayed by the LCD 61. When an image tobe depth-shifted is displayed, the half wave plate is switched off andthe display operates as illustrated in FIG. 10( a). When the nextnon-shifted image is displayed, the half wave plate 69 is switched onand the display operates as illustrated in FIG. 10( c). The half waveplate 69 and the LCD 61 are required to operate at a switching or framerate which is sufficiently high to cause little or no flicker to beperceived by a viewer. Frame rates of between 80 and 120 Hz may be usedwith the actual rate being determined from the brightness of the imageand environmental factors.

Where the mirror 63 reflects approximately 50% and transmitsapproximately 50% of the incident light and the first and second imagesare displayed for substantially equal time periods, the depth-shiftedimage seem by a viewer has a brightness which is approximately onequarter of its original brightness as displayed by the LCD 61, whereasthe non-shifted image has a brightness of about half that displayed bythe LCD 61. The total time-averaged brightness of the display istherefore about ⅜ of the brightness displayed by the LCD 61. However,the display periods of the images and the reflectivity/transmissivity ofthe mirror 64 may be varied to select the relative brightnesses of theimages and the total time-averaged brightness of the display. Forexample, if the depth-shifted image is displayed for twice the durationof the non-shifted image, then the apparent brightnesses of the imagesare substantially equal but the time-averaged brightness becomes onethird of the LCD brightness. Also, if the transmission of the mirror 63is increased above 50%, the increase in brightness of the non-shiftedimage is larger than the decrease in brightness of the depth-shiftedimage so that the overall display brightness increases.

As mentioned hereinbefore, the order of the optical elements may bevaried without changing the way in which the display operates. However,because of variations in the elements from ideal behaviour or ofwavelength-dependencies, there may be a “best” order of the elements inany specific example. Alternative orders of the optical elements whichfunction in slightly different ways are also possible. For example, asshown in FIG. 9( c), the switchable half wave plate 69 may be disposedbetween the mirror 63 and the circular polariser 71. In thisarrangement, light passes only once through the switchable half waveplate 69 so that the effects of any imperfections in its operation arereduced.

As mentioned hereinbefore, any type of transmissive or emissive displaydevice may be used in such displays. By way of example, FIG. 9( c)illustrates the use of a cathode ray tube (CRT) 72 in place of thebacklight 60 and the LCD 61.

Where a liquid crystal cell is used as the switchable half wave plate69, the performance may be improved by the addition of one or morecompensation films. For example, liquid crystal cells may have someresidual retardance when they are nominally switched “off”, generallywhen a voltage is applied across a liquid crystal layer of the cell. Byway of particular example, in the case of a cell which has been designedto switch between zero retardance and 275 nm retardance, there may be aresidual retardance of 50 nm when the cell is switched to providenominally zero retardance. Such residual retardance can cause visiblecrosstalk between the image depth planes. This may be substantiallyremoved by arranging for the cell to have a retardance of 325 nm in the“on” state and by providing a 50 nm fixed retarder in series with thecell and with its fast axis perpendicular to that of the cell. The totalretardance in the “on” state is then the desired 275 nm and the residualretardance is cancelled to provide zero retardance in the “off” state.Further, the actual retardance in both states may be adjusted by varyingthe applied voltages to achieve a desired or optimum performance.

The orientation of some elements about an axis perpendicular to thedisplay plane may be varied from the example given in FIGS. 10( b) and10(d). For example, the quarter wave plate 68 may be rotated by 90° sothat the polarisation state of light on each pass through the quarterwave plate 68 is orthogonal to the states illustrated in FIGS. 10( a)and 10(c). This results in the depth-shifted image being visible whenthe half wave plate 69 is switched on and the non-shifted image beingvisible when the half wave plate 69 is switched off. Also, the circularpolariser 71 may be of the type which transmits right-handed circularpolarization and reflects or absorbs left-handed circular polarization.The half wave plate 69 may be rotated without theoretically affectingperformance but, because of deviations from ideal behaviour orwavelength-dependence, there may be a preferred orientation in practice.

As mentioned hereinbefore, the display device need not be an LCD,although LCDs have the advantage that they emit light which is alreadypolarized so that relatively little light is lost through the reflectivepolariser 62. However, it tends to be more difficult to drive LCDs athigh speed in order to provide a flicker-free time sequential display sothat faster display devices, such as rear projectors, cathode ray tubes,plasma displays and organic LED displays, may also be used.

Although the reflectors 62 and 63 are illustrated as being parallel toeach other and to the image surface of the LCD 61, this is notnecessary. For example, there may be applications where thedepth-shifted and un-shifted images are required to appear to benon-parallel with each other, in which case the reflectors may beoriented appropriately so as to achieve this.

Some of the elements of the display may be made from two or more parts,which may be disposed in different locations from the locationsillustrated in FIGS. 9( a) to 9(c). For example, the circular polariser71 may comprise a quarter wave plate and a linear polariser and thesetwo elements may be separated. For example, the linear polariser may bein the location illustrated for the circular polariser and the quarterwave plate may be below this and separated from the linear polariser bythe glass layer 70 or by the switchable half wave plate 69.

Although the displays illustrated in FIGS. 9( a) to 9(c) and 10(a) to10(d) have been described as providing images for simultaneous viewingat different depth planes, the display may be operated in any of theways described hereinbefore, as with all of the other displays describedherein. For example, an image may be arranged to be switched from oneplane to the other so as to appear to move backwards or forwards. In theexample described hereinbefore for a gaming machine, the image may becontrolled so as to leap forward in order to attract the attention ofviewers when a player wins a prize. Also, the display area may bedivided into two or more segments, which may be controlled independentlyof each other, for example so that the apparent depth of the displayedimage may be selected for each segment independently of each othersegment.

The embodiment illustrated in FIGS. 9( a) to 9(c) and 10(a) to 10(d) iscapable of displaying images in two image planes of different depths.However, the same technique may be used to create more than two imageplanes, all having different depths with respect to the display, andFIG. 11 illustrates an example of this.

The display shown in FIG. 11 comprises elements 60 to 63 and 68 to 71 ofthe same type and in the same relative positions as illustrated in FIGS.9 a and 9 b. The elements 62, 63 and 68 to 71 are referred to in thedrawing as “stack 1” disposed above the LCD 61. A further set ofelements referred to as “stack 2” in FIG. 11 is disposed above stack 1and comprises a reflective polariser 62′, a fixed quarter wave plate68′, a switchable half wave plate 69′, a glass spacer 70′, a “50%”mirror 63′ and a circular polariser 71′. The elements of stack 1 arethus duplicated in stack 2 and are substantially identical except thatthe glass spacer 70′ is of a thickness different from that of the glassspacer 70. This arrangement is therefore capable of displaying images atimage planes of four different depths with respect to the display.

When both of the half wave plates 69 and 69′ are switched on, lightpasses directly through the layers along “path 1” to the viewing regionso that a displayed image is perceived as emanating substantially fromthe actual location of the image-producing plane of the LCD 61. When thehalf wave plate 69′ is switched on and half wave plate 69 is switchedoff, light follows “path 2”, which includes reflections at the mirror 63and the reflective polariser 62. A displayed image appears to be locatedat the image plane 67 b. When the half wave plate 69′ is switched offand the half wave plate 69 is switched on, light follows “path 3”, whichincludes reflections at the mirror 63′ and the reflective polariser 62′.A displayed image is perceived at the image plane 67 a. When both of thehalf wave plates 69 and 69′ are switched off, light follows “path 4”,which includes reflections at the mirror 63, the reflective polariser62, the mirror 63′ and the reflective polariser 62′. A displayed imageis thus perceived at the image plane 67 c.

The display of FIG. 11 may be operated in any of the ways describedhereinbefore in order to achieve a desired effect. For example, bydisplaying four images or sequences of images time-sequentially andcontrolling the switchable half wave plates 69 and 69′ in synchronism,images at four different-depth image planes may be viewed effectivelysimultaneously by a viewer. Alternatively, the image planes may beswitched to give the impression of motion towards and away from theviewer.

The display shown in FIGS. 12( a) and 12(b) comprises a backlight (notshown) and an LCD 61 as the display device although, as in the case ofall of the embodiments described herein, any suitable display device maybe used. In this embodiment, the first partial reflector 62 comprises apartially reflecting mirror which, in this example, is arranged totransmit substantially 50% of incident light and to reflectsubstantially 50% of incident light (ignoring light losses). Also, thesecond partial reflector 63 comprises a cholesteric reflector whichtransmits one circular polarization state (in this case right-handed R)and reflects the orthogonal circular polarization state (in this exampleleft-handed L). The cholesteric reflector 63 comprises a liquid crystallayer with a natural helical structure. Such reflectors are well-knownand are disclosed, for example, in the reference by Lueder mentionedhereinbefore. Such reflectors may be arranged so as to be switched offby the application of an applied electric field or may be fixed andunswitchable, in which case the liquid crystal layer may be fixed bypolymerization.

In the display shown in FIGS. 12( a) and 12(b), the fixed quarter waveplate 68 is disposed between the LCD 61 and the mirror 62. The LCD 61has an exit polariser with a transmission axis 73 and the quarter waveplate 68 is oriented with its fast axis 74 at 45° to the transmissionaxis 73. As in the previous embodiment, the display comprises aswitchable half wave plate 69 between the reflectors 62 and 63 with itsfast axis in any desired orientation.

FIGS. 12( a) and 12(b) illustrates operation in the depth-shifting modeof image display. The linearly polarized light with its electric fieldvector oriented parallel to the plane of the drawing is converted toright-handed circularly polarized light R by the quarter wave plate 68.Approximately half of this light is transmitted by the mirror 62 whereasapproximately half is reflected, converted by the quarter wave plate 68to linearly polarized light with its electric field vector perpendicularto the plane of the drawing, and absorbed by the exit polariser of theLCD 61. The half wave plate 69 is active and converts the transmittedlight to left-handed circularly polarized light L. This polarizationstate is reflected by the cholesteric reflector 63 and converted toright-handed circularly polarized light R. Approximately half of thislight is transmitted by the mirror 62, converted by the quarter waveplate 68 to linearly polarized light with its electric field vectorperpendicular to the plane of the drawing, and absorbed by the exitpolariser of the LCD 61. Half of the incident light on the mirror 62 isreflected and converted to left-handed circularly polarized light L.This is converted to the right-handed circularly polarized state R bythe half wave plate 69 and is transmitted towards the viewing region bythe cholesteric reflector 63.

In the non-depth shifted mode of operation (not illustrated in thedrawings), the half wave plate 69 is switched off and so hassubstantially no effect on the polarisation state of light propagatingthrough it. As before, light emitted by the LCD 61 is converted toright-handed circularly polarized light, which is transmitted withoutany substantial change in polarization by the switched-off half waveplate 69 and is transmitted by the cholesteric reflector 63 towards theviewing region.

In order to provide the desired shift in perceived depth of the image,the reflectors 62 and 63 are spaced apart by the appropriate distanceand this may be adjusted by means of a transparent spacer (not shown),for example made of glass. Also, the order of the elements may bechanged from that illustrated in FIGS. 12( a) and 12(b). For example,the half wave plate 69 may be disposed between the quarter wave plate 68and the mirror 62 or the positions of the mirror 62 and the cholestericreflector 63 may be exchanged. It is further possible to replace thecholesteric reflector 63 with a combination comprising a quarter waveplate and a reflective polariser.

The display illustrated in FIGS. 13( a) and 13(b) differs from thatshown in FIGS. 12( a) and 12(b) in that the switchable half wave plate69 is omitted and the cholesteric reflector 63 is of the switchable typewhich, when switched off, transmits light irrespective of itspolarisation state and, when switched on, transmits left-handedcircularly polarized light L and reflects right-handed circularlypolarized light R. The general arrangement and operation in thedepth-shifted mode is illustrated in FIG. 13 a and the orientation ofthe quarter wave plate and exit polariser of the LCD 61 are illustratedin FIG. 13 b.

In the depth-shifted mode, the cholesteric reflector 63 is switched on.The LCD 61 emits linearly polarized light with its electric field vectororiented in the plane of the drawing. This is converted to right-handedcircularly polarized light R by the quarter wave plate 68, approximatelyhalf of which is transmitted by the mirror 62 without any substantialchange to the polarisation state and approximately half of which isreflected towards the quarter wave plate 68. The quarter wave plate 68converts the reflected light to linearly polarized light with theelectric field vector perpendicular to the plane of the drawing and thislight is absorbed by the exit polariser by the LCD 61.

The light transmitted by the mirror 62 is reflected by the cholestericreflector 63. Half of this light is transmitted by the mirror 68 andabsorbed as described hereinbefore. The reflected portion of the lightis converted to the left-handed circularly polarized state and istransmitted by the reflector 63 towards the viewing region.

In the non-shifted mode, the cholesteric reflector 63 is switched off.The portion of light emitted by the LCD 61 and transmitted by the mirror62 is thus transmitted through the reflector 63 to the viewing region.

Switchable cholesteric reflectors can be made to operate over arelatively narrow band of wavelengths, typically of the order of 100 nm.It may therefore be necessary to embody the switchable cholestericreflector 63 as a stack of such switchable reflectors. For example,three such reflectors may be provided for selectively reflecting theprimary colours red, green and blue. Such reflectors are disclosed, forexample, in “Reflective multicolour displaying using cholesteric liquidcrystals”, M. Okada et al, SID 1997 Digest and “Multiple color highresolution reflective cholesteric liquid crystal displays”, D. Davies etal, SID 1997 Digest. In such an arrangement, the three colours can beswitched independently of each other so that different colours mayoperate in the different modes. By switching the colours between themodes at different times, the visibility of flicker may be reduced.

Displays of this type can be modified so as to show more than two depthplanes. For example, a display of this type capable of showing threedifferent depth planes is illustrated in FIG. 13 c. This display differsfrom that shown in FIG. 13 a in that the switchable cholestericreflector 63 is replaced by two spaced-apart cholesteric reflectors 63 aand 63 b. With both of the reflectors 63 a and 63 b switched off, lightfollows a “direct” path 66 so that substantially no depth-shifting ofthe displayed image takes place. With the reflector 63 a switched on andthe reflector 63 b switched off, the display operates as illustrated inFIG. 13 a and light follows a longer path 65 a to provide a relativelylarge shift in depth of the displayed image. When the reflector 63 b isswitched on and the reflector 63 a is switched off, light follows thepath 65 b to provide a different depth-shifted image plane.

FIGS. 14( a) and 14(b) illustrate a display which is similar to thoseshown in FIGS. 13( a) to 13(c) but in which the switchable cholestericreflector or reflectors are replaced by a pair of fixed “single colour”cholesteric reflectors 63 c and 63 d. The reflectors 63 c and 63 d aredisposed immediately adjacent each other with minimal spacing. Thereflector 63 c reflects red right-handed circularly polarized light andtransmits all other light whereas the reflector 63 d reflects blueright-handed circularly polarized light and transmits all other light.

Green light emitted by the LCD 61 is converted to right-handedcircularly polarized light R by the quarter wave plate 68, approximatelyhalf of which is transmitted and approximately half of which isreflected by the mirror 62. The reflected light returns through thequarter wave plate 68 and is absorbed in the LCD 61 as describedhereinbefore. The transmitted light passes through the reflectors 63 cand 63 d along a direct light path 66 to the viewing region.

Red and blue light emitted by the LCD 61 is similarly converted toright-handed circularly polarized light R by the quarter wave plate 68and is partially transmitted by the mirror 62. The red light isreflected by the reflector 63 c and the blue light is reflected by thereflector 63 d back towards the mirror 62, where part of the light istransmitted and absorbed and part is reflected. The reflected red andblue light has its polarization changed by this reflection to theleft-handed circularly polarized state L and this reflected red and bluelight is transmitted by the reflectors 63 c and 63 d along a light path65.

The light paths 65 and 66 are therefore of different lengths for thedifferent colours. A viewer therefore sees an image with two depthplanes, the deeper image containing red and blue and the shallower imagecontaining green.

The use of red and blue cholesteric reflectors is merely exemplary andreflectors for other combinations of colours may be used. Also, thereflectors may be spaced apart in different planes, for example so thatthe three primary colours are shown in different image planes. Anotherpossibility when using this type of reflector is to switch differentcolours at different times. For example, a dual depth display may switchrapidly between a state A and a state B. In the state A, red and bluecomponents for the “front” depth plane and a green component for the“rear” depth plane are displayed. In the state B, red and bluecomponents for the rear depth plane and a green component for the frontdepth plane are displayed.

FIG. 15( b) illustrates a display of the same general type as that shownin FIG. 14( a) but providing “full colour” images at two differentdepths. The display comprises a cholesteric reflector stack 63, which isarranged to reflect light of the right handed circularly polarized statein a relatively narrow red waveband R1, a relatively narrow greenwaveband G2, and a relatively narrow blue waveband B1. The pixels of theLCD 61 have colour filters for transmitting light in the wavebands R1,R2, G1, G2, B1 and B2 as illustrated in FIG. 15( a). For example, theLCD 61 may include a colour filter arrangement with a repeating pattern,for example of columns or rows for transmitting a repeating pattern suchas R1, G1, B1, R2, G2, B2. The cholesteric reflector stack 63 maycomprise three reflectors, each of which reflects one of the bands of arespective colour. However, it is also possible to provide twocholesteric reflectors having the reflection spectra shown as C1 and C2in FIG. 15( a) so that one reflector reflects the waveband B1 whereasthe other reflector reflects the wavebands G2 and R1.

The LCD 61 displays two images as spatially multiplexed or interlacedstrips with one image being encoded by light in the wavebands R1, G2 andB1 and the other image being encoded by light in the wavebands R2, G1and B2. Light encoding the first such image follows the light path 65whereas light encoding the second such image follows the light path 66.Thus, two full-colour or approximately full-colour images or sequencesare displayed at different perceived depths.

Such a display may be modified for use with a backlight which iscontrollable to illuminate the LCD 61 with light in each of thewavebands in turn. The LCD 61 in such an arrangement does not requireany colour filters and may be operated so as to display the red, greenand blue portions of the two images time-sequentially and in synchronismwith the colour of illumination by the backlight. Alternatively, thecolour filter arrangement illustrated in FIGS. 15( a) and 15(b) may beused with a backlight which supplies light of different red, green andblue wavebands in alternate image frames so that the two images orsequences are displayed time-sequentially in synchronism with operationof the backlight.

The display shown in FIG. 16 differs from the previously describedembodiments in that the optical arrangement disposed in front of the LCD61 comprises reflective polarisers 62 and 63 as the partial reflectorsand a switchable directional half wave retarder 69. The reflectivepolarisers are arranged to transmit light whose electric field vector isoriented in the plane of the drawing and to reflect the orthogonalpolarization state. The switchable directional retarder 69 is switchablebetween an “off” state, in which it has substantially no effect on thepolarization of light passing through it, and an “on” state. In the “on”state, the retarder 69 acts as a half wave plate for light traveling ina direction inclined at +30° to the display plane normal and hassubstantially no effect on the polarization of light traveling at −30°to the display plane normal. The retarder 69 may, for example, comprisea liquid crystal cell of the type disclosed in GB 2 405 516.

FIG. 16 illustrates operation of the display in the depth-shifting modewith the retarder 69 switched on. Viewing conditions are such thatviewers observe the display at an angle close to +30°.

Light emerging from the LCD 61 is linearly polarized with its electricfield vector in the plane of the drawing. The retarder 69 rotates thepolarization of the light passing through it at +30° such that theelectric field vector is perpendicular to the plane of the drawing. Thislight is reflected from the reflective polariser 63 and passes backthrough the retarder 69 at an angle of −30° so that its polarization isunaffected. The light is then reflected by the reflective polariser 62and passes through the retarder 69 at an angle of +30° so that itspolarization is rotated by 90°. The resulting light is transmitted bythe reflective polariser 63 towards the viewing region.

When the retarder 69 is switched off, it has no effect on thepolarization of light passing through it. Thus, the light emitted by theLCD 61 and passing through the polariser 62 also passes through thepolariser 63 as the transmission axes of the reflective polarisers 62and 63 are parallel.

An advantage of this arrangement is that it provides substantiallyfull-brightness images in both modes of operation. In practice, somelosses will occur as light passes through or is reflected by the variousoptical elements. However, no attenuation takes place because of theintended operation of the optical elements.

The display shown in FIGS. 17( a) and 17(b) differs from the previouslydescribed displays in that the optical arrangement in front of the LCD61 comprises reflective polarisers 62 and 63, between which are disposeda fixed Faraday rotator 75 and a switchable half wave plate 69. Thetransmission axis 73 of the polariser 62 is oriented in the plane of thedrawing whereas the transmission axis 76 of the reflective polariser 63is oriented at 45° with respect to the transmission axis 73. Whenswitched on, the half wave plate 69 has a fast axis 77 which isperpendicular to the transmission axis 73. The Faraday rotator rotatesthe polarization of linearly polarized light by +45° upon passage of thelight in either direction through the rotator 75.

Faraday rotators comprise layers of material which rotate thepolarization state of light passing therethrough by an angleproportional to a magnetic field applied to the layer. Such devices areknown and are described in standard reference texts, for example“Optics”, E. Hecht et al, fourth edition, Addison Wesley (2003).

FIGS. 18( a) and 18(b) illustrates operation of the display of FIGS. 17(a) and 17(b) in the depth shifting mode, in which the half wave plate 69is switched off and has substantially no effect on the polarisation oflight passing through it. Linearly polarized light from the LCD 61passing through the reflective polariser 62 has its electric fieldvector oriented in the plane of the drawing. The rotator 75 rotates theelectric field vector so that it is oriented at +45° and is reflectedback by the reflective polariser 63. The light passes through therotator 75 again and emerges with its electric field vector orientedperpendicular to the plane of the drawing. The light is reflected by thepolariser 62 and passes through the rotator 75, which rotates thepolarisation plane so that the electric field vector is oriented at−45°. The reflective polariser 63 transmits this light towards theviewing region.

FIGS. 19( a) and 19(b) illustrate operation in the non-shifting mode. Inthis mode, the half wave plate 69 is switched on. Light with itselectric field vector oriented in the plane of the drawing passesthrough the rotator 75, which rotates the plane of polarisation so thatthe electric field vector is oriented at +45°. The half wave plate 69further rotates the plane of polarisation such that light emerging fromit has its electric field vector oriented at −45°. This light istransmitted by the reflector polariser 63 towards the viewing region.

As in the previous embodiment, light is not lost because of theoperation of the various optical elements. The displayed images aretherefore relatively bright in both the depth-shifted mode and in thenon-shifted mode.

FIGS. 20( a) and 20(b) show a display in which pairs of images to bedisplayed at different effective depths are simultaneously displayed bythe LCD 61 by means of spatial multiplexing. The LCD 61 is pixellatedwith the pixels arranged as rows and columns. The images are displayedas interlaced rows or columns with the “D pixels” displaying, forexample, vertical strips of the depth-shifted image and the “T pixels”displaying vertical strips of the non-shifted image. Alternatively, achequerboard pattern may be used. The LCD has entrance and exitpolarisers 80 and 81 with the exit polariser 81 having a transmissionaxis 83 oriented in the plane of the drawing.

A patterned quarter wave retarder 82 is disposed above the exitpolariser 81 and comprises quarter wave retarder regions whose fast axesalternate with each other. Thus, the retarder portions such as 84 abovethe D pixels have their fast axes oriented so as to convert the lightemitted from the D pixels to right-handed circularly polarized light R.Retarder portions such as 85 disposed above the T pixels have their fastaxes oriented such that light emerging from the T pixels is converted toleft-handed circularly polarized light L.

A “50%” mirror 62 is disposed above the patterned quarter wave retarder82. A quarter wave plate 68 is disposed above the mirror 62 and has afast axis 74 oriented at 45° to the transmission axis 83 of the exitpolariser 81. A reflective polariser 63 is disposed above the quarterwave plate 68 with its transmission axis 76 parallel to the transmissionaxis 83.

Light from the D and T pixels passes through the exit polariser 81 andthe quarter wave plate 82 so as to be converted into right-handed andleft-handed circularly polarized light, respectively. Approximately halfof the light is reflected by the mirror 62 and effectively lost from thesystem. Approximately half of the incident light is transmitted by themirror 62 to the quarter wave plate 68. The light from the D pixels isconverted into linearly polarized light with its electric field vectororiented perpendicular to the plane of the drawing. This light isreflected by the reflective polariser 63 back through the quarter waveplate 68, where the polarisation is converted to the right-handedcircularly polarized state R. Part of the light from the quarter waveplate is transmitted by the mirror 62 and effectively lost to thesystem. Part of the light incident on the mirror 62 is reflected andconverted to the left-handed circularly polarized state L. The quarterwave plate 68 converts this light to the linearly polarized state withits electric field vector parallel to the plane of the drawings and thislight is transmitted by the polariser 63 to the viewing region.

The light from the T pixels transmitted by the mirror 62 is converted bythe quarter wave plate 68 to linearly polarized light with its electricfield vector parallel to the plane of the drawing. The light istherefore transmitted by the reflective polariser 63 towards the viewingregion.

Any substantial separation between the plane within the LCD 61 where theimages are displayed and the patterned retarder 82 results in parallaxeffects, which limit the viewing region for correct viewing of thedisplay and hence the viewer freedom of movement. For example, if theimages are displayed as interlaced columns, with the regions 84 and 85extending in the column direction, a viewer may move up and down whilecorrectly perceiving the displayed images. However, viewer movement fromside to side results in crosstalk between the interlaced images so thatthe multiple depth effect is compromised. Similarly, if the images aredisplayed as interlaced rows, the viewer may move from side to sidewhile still correctly perceiving the displayed images but movement upand down again leads to crosstalk.

FIG. 21( a) shows a modified display which differs from that shown inFIG. 20 in that the backlight 60 is of the collimated type and adiffuser 90 is disposed between the patterned retarder 82 and the mirror62. The collimated backlight 60 directs light through the LCD 61substantially normally to the image plane. Collimated backlights arewell-known, for example as disclosed in “Proceedings of theInternational Display Workshops”, Fukuoka, Japan, December 2004, paperFMC 10-4. Suitable backlights are available, for example, from Omron.

The diffuser 90 is required in order to allow the displayed images to beviewed from a relatively wide viewing range. The diffuser 90 is of atype which has no substantial effect on the polarisation of lightpassing therethrough. An example of such a diffuser is known as a “GRINfilm” and is available, for example, from Microsharp Corporation, UK.

It is also possible to reduce the undesirable effects of parallax byreducing the spacing between the patterned retarder 82 and the imageplane of the LCD 61. For example, the patterned retarder 82 and the exitpolariser 81 may be disposed within the LCD 61 between the LCDsubstrates. Alternatively, the LCD substrate adjacent the polariser 81and the retarder 82 may be made relatively thin and the polariser 81 maybe formed on the substrate or on a relatively thin substrate.

FIG. 21( b) illustrates another modified display which differs from thatshown in FIG. 21( a) in that the exit polariser 81, the patternedretarder 82 and the diffuser 90 are omitted and a quarter wave plate 91is disposed between the LCD 61 and the mirror 62. Because of the absenceof an exit polariser, the individual pixels of the LCD 61 control thepolarisation state of the modulated light so that the LCD 61 controlsthe depth plane in which the pixels appear. When a pixel is switched on,the polarisation state is rotated so that light passing through thepixel passes directly through the display and the image or sequence ofimages using such light has an image depth substantially at the displaysurface of the LCD 61. Conversely, for pixels which are switched off,the modulated light follows the longer light path 65 so that thecorresponding image or images appear at a shifted image plane. In thisarrangement, the LCD 61 determines the pixel image depths but does notmodulate light with the images. The appearance of the display istherefore of two image planes with the image shown at one plane being a“negative” of the image shown at the other plane. Such a display isstriking in appearance any may have applications in advertising andinformation display. For example, information may be shown as brightdetail on a dark background in a “front” plane so that the detailappears to cast a shadow onto a “rear” plane.

In order to display fully controlled images, a further spatial lightmodulator such as an LCD may be disposed between the entrance polariser80 and the collimated backlight 60. The further LCD (not shown) hasentrance and exit polarisers and the same pixel arrangement as the LCD61 with the pixels of the LCDs being aligned. The exit polariser of thefurther LCD may be provided by the entrance polariser 80 shown in FIGS.21( a) and 21(b).

FIG. 22 illustrates a modification to the display shown at the left partof 21(a) and 21(b). According to this modification, the 50% mirror 62and the diffuser 90 are replaced by a combined element formed on asubstrate 101. The element comprises an array of lenses 102 embedded inthe substrate 101 with the lenses 102 having a focal plane whichcoincides with the upper surface of the substrate 101. The upper surfaceof the substrate 101 is coated with a highly reflective mirror layerforming the reflective element 62. Gaps 103 are formed in the mirrorlayer at the focal points of the lenses 102 in the case of sphericallyconverging lenses. Alternatively, the lenses 102 may comprisecylindrically converging lenses with the gaps 103 being formed at thefocal lines of the lenses. The gaps thus occupy only a relatively smallportion of the upper surface area of the substrate 101.

When collimated light is incident on the combined element from thecollimated backlight via the LCD as illustrated at 104, it is focused bythe lenses 102 through the gaps 103 so as to be de-collimated andtransmitted as illustrated at 105. Conversely, when light reflected fromthe reflective polariser 63 is incident on the mirror layer 62 asillustrated at 106, most of the incident light is reflected.

The use of such a combined element provides a substantial improvement tothe efficiency of light utilization and hence brightness of the display.With the display illustrated in FIG. 21( a) using the 50% mirror 62,ignoring losses which occur in practical embodiments, the brightness ofthe non-shifted image is reduced to about 50% of the image brightnessprovided by the LCD 61 whereas the brightness of the depth-shifted imageis reduced to about 25% of the displayed image brightness. By using thecombined element illustrated in FIG. 22, the collimated light 104 istransmitted with relatively high efficiency, for example about 90%.Similarly, light 106 reflected by the mirror layer 62 is reflected withrelatively high efficiency, again about 90%. The brightness of thenon-shifted image is therefore about 90% of the brightness of the imagedisplayed by the LCD 61 whereas the brightness of the depth-shiftedimage is about 81%.

Patterned retarders, such as the retarder 82 shown in FIG. 20 andvarious retarders shown in the embodiments described hereinafter, may bemade in a variety of ways and several of these are illustrated in FIGS.23( a) to 23(e). For example, as shown in FIG. 23( a), the retarderfunction may be destroyed in regions which are required to provide noretarder function. Although FIG. 23( a) to 23(e) refer to half waveretarders, the same principals may be used for quarter wave or otherretarders. Thus, regions such as 107 retain their retarder functionwhereas regions such as 108 have the retarder function destroyed, forexample by exposure to ultraviolet (UV) radiation. Thus, an initiallyuniform retarding layer is formed on a transparent substrate 109 and isthen exposed to ultraviolet irradiation via a suitable mask to providethe patterned retarder. As an alternative, the regions which arerequired not to have a retarder function may be removed as illustratedin FIG. 23( b). Examples of methods of removing the retarder materialinclude etching and laser machining.

Patterned retarders may also be formed using liquid crystal materialsand a first example is illustrated in FIG. 23( c). The patternedretarder is formed between glass substrates 110 and comprises strips ofliquid crystal material 111 interlaced or alternating with strips of atransparent resin or photoresist material 112. The facing surfaces ofthe glass substrate 110 are provided with appropriate alignment layersin order to align the birefringence axes in the desired direction. Suchalignment layers (not shown) may be of any suitable type and thealignment direction may be provided by a known alignment layer rubbingprocess or any other suitable technique.

Where the patterned retarder is required to rotate the linearpolarisation electric field vector of light, a polarisation-rotatingarrangement may be used instead of a retarder. For example, the liquidcrystal material 111 and the adjacent alignment layers may be arrangedto provide a twisted nematic cell for providing polarisation rotation of90° or of any other desired angle of rotation.

The patterned layer may need to be switchable between a patterned and anon-patterned configuration and an example of how this may be achievedis illustrated in FIG. 23( d). In this case, the lower substrate isprovided with a uniform transparent electrode 113, for example made ofindium tin oxide (ITO), whereas the upper substrate has a patternedelectrode 112 formed of the same material. A liquid crystal layer 111 isprovided between alignment layers formed on the electrodes. The liquidcrystal mode employed by the device is such that, when no voltage isapplied between the uniform electrode 113 and the patterned electrodes112, the liquid crystal cell acts as a uniform half wave or otherretarder. When a voltage is applied between the electrodes, the liquidcrystal molecules adjacent the patterned electrodes align substantiallyperpendicularly to the plane of the liquid crystal layer so that theseregions have substantially no retardation, whereas the parts of theliquid crystal layer adjacent the gaps between the patterned electrodeare substantially not affected and continue to provide the desiredretardation.

The arrangement illustrated in FIG. 23( d) may also be used to provide aswitched patterned polarization rotator. For example, in the absence ofan applied voltage between the electrodes, the liquid crystal may bealigned to be in the twisted nematic mode so as to act as a polarizationrotator. When a voltage is applied between the electrodes, the twistednematic structure is lost in the regions of the patterned electrodes 112so as to form a patterned polarization rotator.

In embodiments where a patterned retarder or polarization rotator isrequired across the whole of the area of the element, the optic axes maybe patterned as illustrated in FIG. 23( e). In order to achieve this,either or both of the alignment layers may be patterned. In this case,patterned upper and lower alignment layers 114 and 115 are provided withthe same patterning of alignment directions so that liquid crystalregions A are aligned in the same direction and liquid crystal regions Bare aligned in a different direction. Methods of making such patternedalignment layers are known and examples are disclosed, for example, inEP 0 887 667.

Such a patterned retarder may be used, for example, in the display shownin FIG. 21( a) where the patterned retarder is required to providedifferent retardation or polarization rotation across its whole area.However, a retarder of the type shown in FIG. 23( e) may also be used insome embodiments where only some of the regions are required to provideretardation. For example, if the patterned retarder is a half waveplate, then regions which are not required to provide retardation may beoriented such that their optic axes are parallel or perpendicular to theelectric field vector of linearly polarized light whereas the optic axesof regions required to provide retardation are oriented at a differentangle as appropriate for the specific application.

In the case of patterned retarders using liquid crystal material as the“active” retardation element, it is possible to switch off the dual (ormultiple) effect by making use of the electrically switchable opticaleffect of the liquid crystal material. For example, in the exampleillustrated in FIG. 23 c, both of the substrates 110 may be providedwith uniform transparent electrodes. The application of an electricfield across the liquid crystal material 111 causes the liquid crystalmolecules to align substantially perpendicularly to the plane of theelement so that the liquid crystal material has substantially no effecton the polarization state of normally incident light. The whole area ofthe element then has substantially no effect on polarization so that allof the pixels display an image in the same plane. This may be thenon-shifted image plane because this provides a brighter image in mostembodiments. The example illustrated in FIG. 23( d) has the capabilityof single plane image display because, in the absence of any appliedelectric field, the whole of the liquid crystal layer 111 is in the samestate. The example shown in FIG. 23( e) may also be provided with thiscapability by providing uniform electrodes on both of the substrates 110so as to align all of the liquid crystal molecules substantiallyperpendicular to the plane of the layer so as to provide a uniformlayer.

In embodiments using liquid crystal material to provide patternedretarders (or polarization rotators) without requiring the ability to beswitched to single depth display, the liquid crystal material may befixed during manufacture so as to avoid the need for applying anelectric field across the liquid crystal material. For example, theliquid crystal material may comprise a polymerisable liquid crystalmaterial such as a reactive mesogen made by Merck. Such materials may bepolymerized during manufacture so as to reduce the sensitivity of theliquid crystal cell to humidity, temperature and mechanical damage.

In displays of the types shown in FIGS. 20( a), 20(b), 21(a) and 21(b),in the case of dual depth displays, each of the images is displayed athalf the actual spatial resolution of the LCD 61. This may result inviewers being able to perceive strips in the displayed images. Thevisibility of such strips may be reduced by interlacing different colourcomponents of the images separately. For example, the even rows of adisplay may display red and green components of the non-shifted imageand the blue component of the depth-shifted image whereas the odd rowsof the display may display red and green components of the depth-shiftedimage and the blue component of the non-shifted image. This may beachieved, for example, by inserting into the elements of the display awavelength-dependent retarder, which acts as a half wave retarder forred and green but has substantially no effect for blue. An example ofsuch a wavelength-dependent retarder is disclosed in U.S. Pat. No.6,273,571.

The display shown in FIGS. 24( a) and 24(b) differs from that shown inFIGS. 20( a) and 20(b) in that the patterned quarter wave retarder 82 isreplaced by a uniform quarter wave plate 120 and a patterned half waveretarder 121. The fast axis 123 is perpendicular to the fast axis 74 ofthe quarter wave plate 68. The patterned retarder 121 comprises regionssuch as 124, which have substantially no effect on the polarizationstate of light, and regions such as 125 acting as half wave retardersand having fast axes 126 which are substantially parallel to the fastaxis 123 of the quarter wave plate 120.

The combination of the quarter wave plate 120 and the patterned halfwave retarder 121 in FIGS. 24( a) and 24(b) has substantially the sameeffect on the polarization state of light as the patterned quarter waveretarder 82 in FIGS. 20( a) and 20(b). Thus, light passing through theregions such as 125 of the retarder 121 follows the light path 65whereas light passing through the regions such as 124 follows the lightpath 66. The patterned retarder 121 may comprise any of the elementsdescribed hereinbefore, for example as shown in FIG. 23.

FIGS. 25( a), 25(b) and 26 illustrate a display of a type similar tothat shown in FIGS. 24( a) and 24(b). In this embodiment, the patternedhalf wave plate 121 is embodied by a liquid crystal device of the typeillustrated in FIG. 23( d). Black mask regions 128 are provided on thepatterned retarder 121 and on the LCD 61 so as to prevent undesirableartefacts from being visible. In particular, such artefacts may occur atthe edges of the patterned electrodes 112 and the black mask portionshide this from view. A compensation film 129 is disposed above thepatterned retarder 121 to improve the viewing angle properties of thedisplay. Such compensation films are well known in liquid crystaltechnology and examples are described in the reference by Luedermentioned hereinbefore.

The display further differs from that shown in FIGS. 24( a) and 24(b) inthat the 50% mirror 62 is replaced by a 70% mirror, the orientation ofthe transmission axis 76 of the reflective polariser 63 is rotated by90°, and a further absorbing polariser 130 is disposed above thereflective polariser 63 with its transmission axis parallel to the axis76. The absorbing polariser 130 reduces the reflection of ambient lightfrom the surface of the display. In the absence of such a polariser,incident light whose polarisation is orthogonal to the transmission axisof the reflective polariser 63 is reflected back, for example towards aviewer. The presence of the absorbing polariser 103 prevents or verygreatly attenuates such reflected light.

The mirror 62 is arranged to transmit approximately 70% of incidentlight and to reflect approximately 30% of incident light. Such a mirrorimproves the overall image brightness provided by the display. Inparticular, with a 50% mirror, the non-shifted image has a brightnesswhich is theoretically (ignoring losses) equal to 50% of the imagebrightness at the LCD 61 whereas the depth-shifted image brightness istheoretically reduced to 25%. The use of the 70% mirror increases thenon-shifted image brightness to 70% whereas the depth-shifted imagebrightness is reduced by a relatively small amount to 21%.

The display is illustrated in the dual depth mode in FIGS. 25( a) and25(b) with a voltage applied between the uniform electrode 113 and thepatterned electrodes 112 of the retarder 121. In the liquid crystalregion adjacent the patterned electrodes 112, the liquid crystalmolecules are oriented by the applied field so as to be substantiallyperpendicular to the plane of the liquid crystal layer and thus havesubstantially no effect on the polarization state of light passingthrough such regions. The liquid crystal regions adjacent the gapsbetween the patterned electrodes 112 do not receive an applied field andtherefore remain oriented by the alignment layers. These regions of theliquid crystal material thus act as half wave plates with their fastaxis oriented parallel to the fast axis 123 of the quarter wave plate120. The light passing through these regions from the D pixels thuspropagates directly to the viewing region whereas light from the Tpixels propagating through the “switched” liquid crystal regions followsthe longer path 65 to the viewing region.

As described hereinbefore, the alignment of the liquid crystal cell maybe such that, in the absence of an applied field, the liquid crystalmaterial is in the twisted nematic mode and acts as a polarizationrotator for rotating the electric field vector of linearly polarizedlight by 90°. The operation of the display is thus as described abovebecause the twisted nematic structure is destroyed by the applied fieldin the liquid crystal regions adjacent the patterned electrodes 112.

FIG. 26 illustrates the display of FIGS. 25( a) and 25(b) in analternative single depth mode of operation. In this mode of operation,no electric field is applied between the electrodes 112 and 113 so thatthe liquid crystal mode is determined by the alignment layers and theliquid crystal material. In this case, the element 121 acts as a uniformhalf wave plate or polarization rotator so that all of the lightmodulated with the image or sequence of images by the LCD 61 follows adirect path 66 to the viewing region and the image is displayed as anon-shifted image.

In a variation of the device shown in FIGS. 25( a) and 25(b), a liquidcrystal cell with patterned alignment as shown in FIG. 23( e) is used.The liquid crystal regions A-B-A-B in FIG. 23( e) act as quarter waveplates of alternating orientation, and the quarter wave plate 120 is notnecessary. If a compensation film 129 is also not used, then thefunctions of the mirror 62 and black mask 128 may be combined by makingthe black mask 128 from reflecting material such as aluminum or silver.

FIGS. 27( a) and 27(b) illustrate a modified form of the display shownin FIGS. 25( a), 25(b) and 26. In particular, the black mask 128 and thecompensation film 129 are omitted. The operation of the display of FIGS.27( a) and 27(b) is substantially the same as that of the display ofFIGS. 25( a), 25(b) and 26 and will not be described again.

FIG. 28 illustrates a display which is arranged to permit an increase inimage brightness. The display differs from that shown in FIG. 25( a) inseveral respects. The quarter waveplate 120 is disposed between theliquid crystal cell 111 and the exit polariser 81. The part of the blackmask 128 on the liquid crystal cell 111 performs the function of thepartial mirror 62 and comprises a patterned metallic reflector, forexample in the form of a thin layer of aluminum (Al) or silver. Byarranging the reflective parts of the partial mirror to correspond tothe non-transmissive black mask regions 128 in the LCD 61, an overallbrightness increase may be achieved as the non-transmissive parts of themirror 62 obscure regions where little or no light would pass. In thisconfiguration, the liquid crystal cell 111 is required to function as ahalf waveplate, and not in a guiding mode, and may be embodied as aFreedericksz cell.

FIG. 29 shows a display which differs from that shown in FIG. 28 in thata metallic mirror of spatially varying reflective density acts as themirror 62. In this example, there are reflective regions 62 a and 62 bof different reflective/transmissive densities. The density may bevaried, for example, by thinning the reflective layer or by spatialpatterning on a small scale. Such an arrangement may be useful inreducing Moire fringes caused by regular patterning beating with otherspatially varying structures within the display.

FIG. 30 illustrates a display of a type similar to that shown in FIG. 28but in which the function of the quarter waveplate 68 is incorporatedwithin the liquid crystal cell. The cell is spatially patterned withtransparent polymer steps 111 a. The steps such as 111 d extendthroughout the thickness of the cell whereas the steps such as 111 eextend upwards (with the orientation shown in FIG. 30) from the lowersurface and have the reflective regions of the reflector 62 formed ontheir upper surfaces. The regions of the cells above the steps 111 e andthe reflective regions have a thickness such that the liquid crystalmaterial acts as a quarter waveplate 111 c. The thicknesses or depths ofthe regions 111 e and the reflective regions on top thereof have athickness such that they define a liquid crystal cell providing a halfwaveplate 111 b.

Such an arrangement requires a reduced number of individual components,which may be advantageous in at least some applications. Also, the lightpath from the “upper” or front pixels is not required to pass throughany waveplates and so experiences reduced losses and reduced crosstalkresulting from polarization errors. However, in an alternativeembodiment, the transparent polymer regions 111 d may be replaced byliquid crystal material and electrodes for applying a suitable voltageacross the liquid crystal material.

FIG. 31 illustrates a display which differs from that shown in FIG. 28in that the quarter waveplate 68 comprises regions disposed above thepatterned aluminum regions forming the partial reflector 62. The quarterwaveplate regions 68 are therefore only traversed by light following thepath 65, so that losses and crosstalk resulting from polarisation errorsmay be reduced.

In the embodiments described hereinbefore, the front and back (or upperand lower) images are displayed with substantially the same spatialresolution, for example with strips of alternate images being displayedby alternate rows of pixels of the LCD 61. However, this is notnecessary and the upper and lower images may be displayed with differentspatial resolutions. FIG. 32 illustrates an example of this for adisplay of the type shown in FIG. 27( a). In this case, twice as manypixels in twice as many rows are allocated to the lower image as to theupper image. Such as arrangement provides higher resolution and greaterbrightness for the lower image. However, the numbers of pixels and thenumbers of rows may be allocated as desired for any particularapplication.

FIG. 33 illustrates a display which differs from that shown in FIG. 27(a) in that a half waveplate 121 is disposed between the quarterwaveplate 120 and the liquid crystal cell 111. The combination of thequarter waveplate 120 and the half waveplate 121 performs substantiallythe same function as the quarter waveplate 120 on its own in the displayof FIG. 27( a). However, the waveplate combination may provide a moreachromatic response and this may be useful in reducing crosstalk andcolour artefacts.

Various undesirable artefacts may occur in the displays and may resultin reduced performance. For example, Fresnel reflections may occur atinterfaces between components of the display. Such reflections may occurat the interfaces between the quarter waveplate 68 and the partialmirror 62, between the partial mirror 62 and the quarter waveplate 120,and between the quarter waveplate 120 and the liquid crystal cell 111 inthe display shown in FIG. 32. Such reflections may introduce losses andincrease crosstalk. These reflections may be reduced, for example, byindex-matching between components or by applying anti-reflectioncoatings to one or more of the component surfaces.

Typical backlights for the displays are generally designed to provideuniform illumination over a relatively wide angular range to provide alarge viewing area. However, the displays disclosed herein generallyhave a more limited angular viewing range because of parallax betweenthe liquid crystal cell and other components of the display. It istherefore possible to use a partially collimated backlight providing anarrower angular range of illumination of the display. For a given inputpower, such an arrangement provides a higher image brightness.

In at least some of the displays disclosed herein, an alternative liquidcrystal mode may be used. In particular, instead of a conventionaltwisted nematic configuration, a thicker layer of liquid crystal may beused such that it operates in the known Mauguin region. Such anarrangement may give lower polarization errors, both on-axis and athigher angles, and may thus reduce crosstalk.

In order to increase brightness in the “single image mode” whereavailable, the partial mirror 62 may be electrically switchable to atransparent non-reflecting mode. An electrically switchable mirror ofsuitable type is disclosed, for example, in U.S. Pat. No. 6,961,105.

FIG. 34 illustrates how a display of the type disclosed herein may bemounted in a housing in the form of a casing 131. In the arrangementshown in FIG. 34, the components 80, 61, 81, 111, 112, 120, 62 and 68are mounted within the casing 131. The reflective polariser 63 and theabsorbing polariser 130 are mounted outside the casing above atransparent window so that the casing provides a convenient means forspacing the reflective polariser 53 from the partial mirror 62 toprovide the increased depth effect.

FIG. 35 illustrates an image processing technique which may be used withthe displays disclosed herein to increase the apparent brightness of thedisplay. This may be particularly useful for the image formed in the“lower plane”. The technique relies on adjusting the gamma values in theimage. The gamma values refer to the correspondence between the greylevel data in the image and the actual voltages applied to the LCD andhence the brightness that is ultimately displayed. FIG. 35 illustratesthe displayed brightness against grey scale value and illustrates alinear relationship. FIG. 35 also illustrates an “increased gamma”function such that the displayed brightness of the mid-grey levels areincreased whereas the fully black and fully white levels remainunchanged. This results in an increase in the apparent brightness in theimage.

FIG. 36 shows at 140 a typical pixel layout of a known LCD panel. Theblack regions are non-transmitting and are covered with a black materialto form a mask covering, for example, transistors, capacitors, andcontrol lines of the panel. All of the pixels in this arrangement havethe same aperture ratio. Such a pixel arrangement may be used in thedisplays disclosed herein. However, the layout may be modified asillustrated at 141 so that the pixels L for displaying the lower planeimage have a larger aperture ratio than the pixels U displaying theupper plane image. Such an arrangement allows the brightness of thelower plane image to be increased.

FIG. 37 illustrates another pixel arrangement at 142 with theconventional arrangement being again shown at 140. In the arrangement142, the non-transmitting regions between rows of pixels are increasedand the regions between columns of pixels are reduced so as to maintainthe overall aperture ratio. Such an arrangement may be used to increasethe viewing freedom of the display.

FIG. 38 illustrates another possible application of displays of the typedescribed hereinbefore. In this application, the display is switchablebetween a narrow viewing angle or “private” mode and a wide viewingangle or “public” viewing mode. When in the private viewing mode, thedisplay provides a restricted angular viewing range so that a centralviewer 132 can see the displayed image whereas the displayed image isnot visible to an off-axis viewer 133. In the private mode, the displayis operated such that the image is visible in the depth-shifted position67 “behind” the LCD. This mode may be used, for example, to displayfinancial or other delicate or sensitive information which is notintended for public viewing. When information intended for publicviewing is displayed, the display is operated in the non-shifted mode sothat the image appears to be substantially at the display surface of theLCD 61. In order to provide a sufficiently wide angular viewing range inthe public mode, the polarisation optics 62-64 is made wider than thedisplay surface of the LCD 61.

It is usual for the LCDs 61 described hereinbefore to be updated orrefreshed a row at a time starting from the top of the LCD andcontinuing to the bottom so as to refresh a complete frame. In the caseof time-sequentially operated displays where the LCD 61 alternatelydisplays the first and second images or sequences, switching between theimages occurs row by row. An example of the resulting displayed imagesis illustrated in FIG. 39. In this case, the images are changed every 16milliseconds, which is the frame rate of the LCD.

This may cause problem for displays in which one or more elements of theoptical arrangement in front of the LCD is switched in synchronism withthe sequences of images. For example, the half wave plate in theembodiment of FIG. 9( a) is required to be switched on for one of theimages or sequences and switched off for the other image or sequence.

In order to reduce the effects of row and row refreshing of the LCDs,the associated switched optical elements may be divided into a pluralityof individually switchable segments, for example so that each segmentcovers a plurality of rows of the LCD. The individual segments are thenswitched in sequence and in synchronism with switching of all of theunderlying rows of the LCD so that the time difference between therefreshing of any pixel and the switching of the individual segmentabove it is relatively small. This is illustrated in FIG. 40, where thesegments of the optical element switched to one of its states are shownshaded, the optical element having four such segments.

FIGS. 41( a) and 41(b) illustrate such a segmented switchable opticalelement in the form of a liquid crystal cell having three independentlyswitchable horizontal segments 93, 94 and 95. The optical elementcomprises glass substrates 96 and 97 defining therebetween a liquidcrystal cell containing a layer of liquid crystal material 98. The lowersubstrate 97 carries a single electrode extending over the whole area ofthe optical element whereas the upper substrate 96 carries threeelectrodes 99 defining the three sections of the optical element. Theindividual electrodes 99 are separately addressable so as to permitindependent switching of the optical element segments. The electrodes 98and 99 carry suitable alignment layers 100 for the liquid crystal modeof the optical element.

Another technique for reducing the effects of row by row LCD refreshingis to increase the time period between the refreshing of consecutiveframes. This allows the fraction of the frame refresh period used toperform refreshing to be reduced so that synchronization errors causedby row by row updating are reduced.

The effects of row by row updating may also be reduced by means of abacklight 60 which is switched off during refreshing of the rows andswitched on between consecutive frame refreshing periods. Some types ofbacklight, such as cold-cathode fluorescent lamps, do not switch on andoff instantaneously. However, reducing the illumination time to afraction of the frame period reduces synchronization errors and hencecrosstalk between images as perceived by a viewer.

For example, such a backlight may be switched on only during the“waiting period” between consecutive frame refreshes and for a fewmilliseconds at the beginning and end of each frame refresh period. Theor each switchable optical element switches state during the time whenthe backlight is switched off. Although this results in a smallsynchronisation error for pixels close to the top and bottom of thescreen, there is substantially no error for the majority of the pixelsof the LCD 61 so that an improvement in crosstalk performance isprovided.

In all of the embodiments described hereinbefore, it is possible thatthe separation of the images between the different depth image planeswill not be perfect. Some of the image intended for the depth-shiftedplane D may leak into the true-depth plane T and some of the T image mayleak into the plane D. Such leakage results in crosstalk, which shouldbe reduced so as to be substantially imperceptible to a viewer. Forexample, if the T image is a bright figure on a dark background and theD image is mainly dark, a viewer may see a faint trace of the brightfigure superimposed on the D image.

There are a number of reasons why such crosstalk is likely to happenboth from the D image to the T image and from the T image to the Dimage. Polarisation-manipulating optical elements are generally notperfect. For example, practical polarisers generally transmit some ofthe “wrong polarisation”, retarders have behaviours which depend onorientation, wavelength and processing conditions, and (in timesequential displays) liquid crystal elements have finite switchingtimes, resulting in time periods when light appears to come from both ofthe planes T and D.

Most crosstalk mechanisms lead to an approximately linear dependence ofboth the T and D image brightnesses on the original image data. This isbecause doubling the brightness of a particular pixel in the displaydevice at a particular time results in a doubling of both light fromthat pixel in the depth plane for which it is intended and leakage oflight from that pixel at that time into the depth plane for which it isnot intended.

The problem of crosstalk may therefore be represented by a matrixformulation. This will be described hereinafter for a time-sequentialtype of display but similar techniques may be used for displays relyingon interlaced images and wavelength multiplexing. It is assumed that thedata value sent to the display device is proportional to the brightnessdisplayed by its pixels. However, if this is not the case, then theactual “transfer function” must be taken into account. For example,cathode ray tube devices often have a power-law response where thedisplayed brightness is proportional to a power of the voltage at thesignal input.

Let d be a vector whose two components are the data sent to a particularpixel of the display device (CRT, LCD or other device) at times in thetime-sequential imaging cycle when the display is in modes D and Trespectively. Suppose that the range of data values available is between0 and 1. The vector b contains the brightnesses of the images seen bythe viewer in the two depth planes at this particular pixel. Because ofthe linearity mentioned above, the two vectors are related by a 2×2matrix M.

B=Md.

When using the display, it is necessary to specify the brightnesses b′and calculate the data d′ which need to be sent to the display in orderto show those brightnesses. It is therefore necessary to invert thematrix and calculate d′ according to.

d′=M−1b′.

In principle, this calculation adjusts for the crosstalk and allowsundistorted images to be displayed. Unfortunately, it also may lead tovalues of the components of d′ outside the allowed range [0,1]. It isnecessary to use a range of brightnesses b′ which will avoid this.

The components of the matrix are the positive numbers

$M = {\begin{matrix}\begin{bmatrix}a & \beta\end{bmatrix} \\\begin{bmatrix}\gamma & d\end{bmatrix}\end{matrix}.}$

If the brightness of depth plane 1 is controlled mainly by datacomponent d₁, and depth plane 2 by component d₂, then α>β and δ>γ. Allpossible values of the image data d are in the unit square as shown inFIG. 30( a). FIG. 30( b) shows the resulting range of possible values ofb (the quadrilateral outlined with a solid line).

If β<b₁<α and γ<b₂<δ, then both brightnesses can be varied independentlywithout leading to values of d′ outside the allowed range. Thisrestricts the values of b′ to the rectangle B. Because the minimumvalues of the brightnesses are not equal to zero, there is a loss ofcontrast.

In practice, the raw image data d is mapped into the brightness range Bby a scaling operation:

b′ ₁=β+(α−β)d ₁

b′ ₂ =g+(d−γ)d ₂

Corrected data d′ is then calculated by applying the inverted matrix M⁻¹to b′.

FIGS. 42( a) and 42(d) show an example of this process. The lower graphin FIG. 42( c) shows one row of image data sent to a dual-depth displaywithout crosstalk correction. The co-ordinate x measures distance acrossthe screen. In the example, the components of M are:

$M = {\begin{matrix}\begin{bmatrix}0.9 & 0.2\end{bmatrix} \\\begin{bmatrix}0.1 & 0.6\end{bmatrix}\end{matrix}.}$

The upper graph in FIG. 42( c) shows the brightnesses of the images inthe two depth planes. At x=12 there is an edge in the component b1 whichis not present in the data d1. This is caused by crosstalk.

FIG. 42( d) shows the process of crosstalk correction. Brightnesses b′are calculated from the data d by mapping the components into theallowed brightness ranges. Corrected data d′ is then calculated byapplying the inverted matrix M⁻¹ to b′. Unwanted features in the imagescaused by crosstalk are removed, but there is a loss of contrast becausezero brightness is no longer available.

The matrix M may depend upon the colour of light (red, green or blue)and also on the position of a pixel on the display. The crosstalkcorrection may be applied using values of the coefficients which dependon colour and/or on position.

M may also depend on environmental factors. In particular, thetemperature may affect the switching time of liquid crystal cells andtherefore the crosstalk. The display may therefore include environmentalsensors which feed information into the crosstalk correction method,changing the coefficients in response to changes in the environment.

A feedback method may be used to control the coefficients used incrosstalk correction. For example, pixels in one corner of the display(perhaps hidden behind a cover) may be monitored by photodiodes angledso that they can detect light from the two depth planes independently.Brightnesses measured by these photodiodes are then used to correct thecrosstalk correction coefficients.

In some situations, crosstalk may not follow the linear model givenabove, for example so that doubling the intensity of image D does notdouble the intensity of the crosstalk into image T, or so that theintensity of crosstalk into image T depends upon the brightness of imageT. In this situation, crosstalk correction may still be applied butmeasurements of the crosstalk must be made for a number of values ofimage brightness in both plane D and plane T in order to applycorrection which operates well across all brightness values in bothplanes.

FIGS. 43( a) and 43(b) illustrate images in the upper and lower imageplanes for representing a control button and its operation. For example,a touch panel may be provided in front of the display so that theposition on the display which a user has pressed may be detected. FIG.43( a) represents a control button which has not been depressed. The“top” of the button is shown at 155 and is displayed in the upper imageplane whereas the button surround or bezel is displayed at 156 in thelower image plane. FIG. 43 b illustrates the images when the controlbutton has been depressed. In this case, the top of the control button155 and the bezel 156 are both displayed in the lower image plane. Suchan arrangement may be used to provide a more realistic “feeling” of aphysical control button which reacts when being pressed. An example ofan application for such an arrangement is in an automotive centreconsole for controlling an in car entertainment system or satellitenavigation system.

In the embodiments described hereinbefore, an optical system has beenused with a display device in order to provide a display, which iscapable of changing the depth of the image plane or displaying images attwo or more different depth planes. However, the optical system may beused for other purposes. For example, the optical system may be used inorder to reduce the length of optical instruments by providing anoptical or light path which is longer than the physical length of theoptical system.

For example, such a optical system may be used in a telescope or othersimilar instrument. FIG. 44 of the accompanying drawings illustrates aknown type of astronomical or Keplerian telescope. Such a telescopecomprises an objective lens 140 having a focal length of f_(ob) and anocular lens 141 having a focal length f_(oc). The lenses 140 and 141have coincident focal planes 142 and provide an angular magnificationequal to f_(ob)/f_(oc).

In order to provide an high magnification, the focal length f_(ob) ofthe objective lens 140 should be made relatively large and/or the focallength f_(oc) of the ocular lens 141 should be made relatively small.Optical aberrations produced by the ocular lens 141 limit the minimumfocal length f_(oc) to a few lens diameters. Accordingly, in order toobtain relatively high magnifications, the objective lens 140 isrequired to have a long focal length f_(ob). The total length of thetelescope between the lenses is equal to the sum of the focal lengths sothat a high magnification requires a long physical length of thetelescope.

FIG. 45 illustrates an astronomical telescope including an opticalsystem of the type illustrated in FIG. 9( a) and comprising a reflectivepolariser 62, a quarter wave plate 68, a 50% mirror 63 and a circularpolariser 71. This optical system is disposed between the lenses 140 and141 and operates in a single mode so that the optical system providesthe elongate light path 65. By disposing the reflective polariser 62adjacent the objective lens 140 and the circular polariser 71 and themirror 63 adjacent the ocular lens 141, the physical length of thetelescope may be reduced to nearly one third of the length of the knowntelescope illustrated in FIG. 44 for the same magnification. Thus, amuch more compact instrument of the same magnification performance maybe provided.

Any of the optical systems described hereinbefore for producing thelight path 65 may be used between the lenses 140 and 141. Also, theoptical system shown in FIG. 45 and any of the optical systems of theother embodiments may be used in other applications to shorten thelength of an instrument while maintaining the length of the opticallight path. For example, in order to provide a terrestrial telescope orsimilar instrument, the inverter image produced by astronomicaltelescopes of the type shown in FIGS. 44 and 45 has to be inverted andthis is typically achieved by means of a mirror, an additional lens, ora prism. In the terrestrial telescope illustrated in FIG. 46, a mirror143 is used for image inversion. The mirror 143 is inserted into atelescopic system of the type illustrated in FIG. 45 and is disposedbetween the quarter wave plate 68 and the reflective polariser 62. Thisallows a terrestrial telescope of a given magnification to be made muchmore compact.

FIG. 47 illustrates diagrammatically a known type of digital camera. Thecamera comprises a compound lens 145, which forms an image of an object146 at the sensing plane of an image sensor 147. The compound lens 145may be moveable along a longitudinal axis 148 of the camera so as toallow correct focusing of images from objects at different distancesfrom the camera. The distance from the compound lens 145 to the imagesensor 147 is determined at least partly by the focal length of the lens145.

As shown in FIG. 48, the physical length of the camera may be shortenedby disposing an optical system of any of the types describedhereinbefore between the image sensor and the compound lens. By way ofexample, FIG. 48 shows an optical system of the type shown in FIG. 45.The reflective polariser 62 is disposed adjacent the inner end of thecompound lens 145. The mirror 63 and the circular polariser 71 areattached to the front surface of the image sensor 147. It is thuspossible to produce a much “shorter” camera for a given focal length ofthe compound lens 145. Such an arrangement may therefore be used wherecompact dimensions are required, for example in mobile or “cellular”telephones incorporating camera features.

1. An optical system for providing a first light path which is longerthan a physical length of the system, said optical system comprisingfirst and second spaced-apart partial reflectors and providing the firstlight path for a first light incident on the first reflector, the firstlight path comprising at least partial transmission through the firstreflector towards the second reflector, at least partial reflection fromthe second reflector towards the first reflector, at least partialreflection from the first reflector towards the second reflector, and atleast partial transmission through the second reflector, the opticalsystem being arranged substantially to prevent emission from the secondreflector of the first light not reflected during reflection by thefirst and second reflectors, wherein light incident on the secondpartial reflector for the first time does not leave the optical system.2. A system as claimed in claim 1, in which the optical system isarranged to change the polarization of the first light during passagealong the first path.
 3. A system as claimed in claim 2, in which theoptical system is arranged to change the polarization of the first lightduring passage along the first path between incidence on the secondreflector and reflection from the first reflector.
 4. A system asclaimed in claim 1, in which the optical system is arranged to provide asecond light path of length different from that of the first path.
 5. Asystem as claimed in claim 4, in which the second light path comprisesat least partial transmission through the first reflector towards thesecond reflector and at least partial transmission through the secondreflector.
 6. A systems claimed in claim 5, in which the optical systemis arranged substantially to prevent emission from the second reflectorof the second light not transmitted by the second reflector.
 7. A systemas claimed in claim 4, in which the optical system is switchable betweena first mode, in which the first light propagates along the first lightpath, and a second mode, in which light propagates along the secondlight path.
 8. A system as claimed in claim 1, in which the first andsecond reflectors are substantially plane.
 9. A system as claimed inclaim 1, in which the first and second reflectors are substantiallyparallel.
 10. A system as claimed in claim 1, in which the first andsecond reflectors comprise a reflective linear polariser and a partiallytransmissive mirror, respectively, and the optical system comprises: acircular polariser with the second reflector disposed between the firstreflector and the circular polariser; a quarter wave plate disposedbetween the first and second reflectors; and a switchable half waveplate disposed between the first reflector and the circular polariser.11. A system as claimed in claim 1, in which the first and secondreflectors comprise a partially transmissive mirror and at least onereflective circular polariser, respectively.
 12. A system as claimed inclaim 11, in which the optical system comprises a quarter wave plate.13. A system as claimed in claim 12, in which the optical systemcomprises a switchable half wave plate.
 14. An optical instrumentcomprising a system as claimed in claim
 1. 15. An instrument as claimedin claim 14, comprising any one of a telescope, a monocular, a pair ofbinoculars and a camera.
 16. A display comprising a display device formodulating a first light with a first image or sequence of images and anoptical system arranged to increase the perceived depth of the locationof the first image or sequence, the optical system comprising first andsecond spaced-apart partial reflectors and providing a first light pathfor the first light from the device to a viewing region, the first lightpath comprising at least partial transmission through the firstreflector towards the second reflector, at least partial reflection fromthe second reflector towards the first reflector, at least partialreflection from the first reflector towards the second reflector, and atleast partial transmission through the second reflector towards theviewing region.
 17. A display as claimed in claim 16, in which theoptical system is arranged substantially to prevent transmission to theviewing region of the first light not reflected during reflection by thefirst and second reflectors
 18. A display as claimed in claim 16, inwhich the optical system is arranged to change the polarization of thefirst light during passage along the first path.
 19. A display asclaimed in claim 18, in which the optical system is arranged to changethe polarization of the first light during passage along the first pathbetween incidence on the second reflector and reflection from the firstreflector.
 20. A display as claimed in claim 16, in which the device isarranged to modulate a second image or sequence of images and theoptical system is arranged to provide a second light path from thedevice to the viewing region of length different from that of the firstpath to provide a perceived depth of location of the second image orsequence different from that of the first image or sequence.
 21. Adisplay as claimed in claim 20, in which the second light path comprisesat least partial transmission through the first reflector towards thesecond reflector and at least partial transmission through the secondreflector towards the viewing region.
 22. A display as claimed in claim21, in which the optical system is arranged substantially to preventtransmission to the viewing region of the second light not transmittedby the second reflector.
 23. A display as claimed in claim 20, in whichthe display is switchable between a first mode displaying the firstimage or sequence and a second mode displaying the second image orsequence to change the perceived depth of image location.
 24. A displayas claimed in claim 20, in which the display is arranged to display thefirst and second images or sequences simultaneously or time-sequentiallyto give the appearance of one of the first and second images orsequences overlaid above the other of the first and second images orsequences.
 25. A display as claimed in claim 16, in which the first andsecond reflectors are substantially plane.
 26. A display as claimed inclaim 16, in which the first and second reflectors are substantiallyparallel.
 27. A display as claimed in claim 26, in which the first andsecond reflectors are substantially parallel to a display surface of thedevice.
 28. A display as claimed in claim 16, in which the devicecomprises a liquid crystal device.
 29. A display as claimed in claim 16,in which the first and second reflectors overlie substantially the wholeof an image displaying region of the device.
 30. A display as claimed inany one of claim 16, in which the first and second reflectors comprise areflective linear polariser and a partially transmissive mirror,respectively, disposed between the device and a circular polariser andthe optical system comprises a quarter wave plate disposed between thefirst and second reflectors and a switchable half wave plate disposedbetween the first reflector and the circular polariser.
 31. A display asclaimed in claim 16, in which the first and second reflectors comprise apartially transmissive mirror and at least one reflective circularpolariser, respectively.
 32. A display as claimed in claim 31, in whichthe optical system comprises a quarter wave plate.
 33. A display asclaimed in claim 32, in which the optical system comprises a switchablehalf wave plate.
 34. A display as claimed in claim 16, in which thefirst and second reflectors comprise a partially transmissive mirror anda reflective linear polariser, respectively, and the optical systemcomprises a quarter wave plate disposed between the first and secondreflectors and a patterned retarder or polarization rotator disposedbetween the first reflector and the device.
 35. A display as claimed inclaim 34, comprising a collimated backlight and in which the opticalsystem comprises a diffuser.
 36. A display as claimed in claim 35, inwhich the partially transmissive mirror comprises a mirror having anarray of apertures and the diffuser comprises an array of lenses alignedwith the apertures.
 37. A display as claimed in claim 33, in which thepatterned retarder or rotator is switchable to a uniform unpatternedstate.
 38. A display as claimed in claim 33, in which the patternedretarder or rotator comprises a patterned quarter wave plate.
 39. Adisplay as claimed in claim 34, in which the patterned retarder orrotator comprises a uniform quarter wave plate and a patterned half waveplate or patterned 90° polarisation rotator.
 40. A display as claimed inclaim 34, in which the patterned retarder or rotator comprises a liquidcrystal cell.
 41. A display as claimed in claim 40, in which the liquidcrystal cell comprises a patterned electrode arrangement.